diff --git a/src/cache.rs b/src/cache.rs
index 966462a..3702b73 100644
--- a/src/cache.rs
+++ b/src/cache.rs
@@ -14,7 +14,6 @@ use async_trait::async_trait;
 use chrono::{DateTime, Utc};
 
 use crate::traits::CalDavSource;
-use crate::traits::SyncSlave;
 use crate::traits::PartialCalendar;
 use crate::traits::CompleteCalendar;
 use crate::calendar::cached_calendar::CachedCalendar;
@@ -192,16 +191,6 @@ impl CalDavSource<CachedCalendar> for Cache {
     }
 }
 
-impl SyncSlave for Cache {
-    fn get_last_sync(&self) -> Option<DateTime<Utc>> {
-        self.data.last_sync
-    }
-
-    fn update_last_sync(&mut self, timepoint: Option<DateTime<Utc>>) {
-        self.data.last_sync = Some(timepoint.unwrap_or_else(|| Utc::now()));
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/src/calendar/cached_calendar.rs b/src/calendar/cached_calendar.rs
index f0c4f9e..4376846 100644
--- a/src/calendar/cached_calendar.rs
+++ b/src/calendar/cached_calendar.rs
@@ -10,6 +10,7 @@ use crate::traits::{PartialCalendar, CompleteCalendar};
 use crate::calendar::{CalendarId, SupportedComponents, SearchFilter};
 use crate::Item;
 use crate::item::ItemId;
+use crate::item::VersionTag;
 
 
 /// A calendar used by the [`cache`](crate::cache) module
@@ -69,6 +70,32 @@ impl PartialCalendar for CachedCalendar {
         Ok(())
     }
 
+    async fn get_item_version_tags(&self) -> Result<HashMap<ItemId, VersionTag>, Box<dyn Error>> {
+        Ok(self.items.iter()
+            .map(|(id, item)| (id.clone(), item.version_tag().clone()))
+            .collect()
+        )
+    }
+
+    async fn get_item_by_id_mut<'a>(&'a mut self, id: &ItemId) -> Option<&'a mut Item> {
+        self.items.get_mut(id)
+    }
+}
+
+#[async_trait]
+impl CompleteCalendar for CachedCalendar {
+    /// Returns the items that have been deleted after `since`
+    async fn get_items_deleted_since(&self, since: DateTime<Utc>) -> Result<HashSet<ItemId>, Box<dyn Error>> {
+        Ok(self.deleted_items.range(since..)
+            .map(|(_key, id)| id.clone())
+            .collect())
+    }
+
+    /// Returns the list of items that this calendar contains
+    async fn get_items(&self) -> Result<HashMap<ItemId, &Item>, Box<dyn Error>> {
+        self.get_items_modified_since(None, None).await
+    }
+
     async fn get_items_modified_since(&self, since: Option<DateTime<Utc>>, filter: Option<SearchFilter>) -> Result<HashMap<ItemId, &Item>, Box<dyn Error>> {
         let filter = filter.unwrap_or_default();
 
@@ -96,28 +123,4 @@ impl PartialCalendar for CachedCalendar {
 
         Ok(map)
     }
-
-    async fn get_item_ids(&self) -> Result<HashSet<ItemId>, Box<dyn Error>> {
-        Ok(self.items.keys().cloned().collect())
-    }
-
-    async fn get_item_by_id_mut<'a>(&'a mut self, id: &ItemId) -> Option<&'a mut Item> {
-        self.items.get_mut(id)
-    }
-}
-
-#[async_trait]
-impl CompleteCalendar for CachedCalendar {
-    /// Returns the items that have been deleted after `since`
-    async fn get_items_deleted_since(&self, since: DateTime<Utc>) -> Result<HashSet<ItemId>, Box<dyn Error>> {
-        Ok(self.deleted_items.range(since..)
-            .map(|(_key, id)| id.clone())
-            .collect())
-    }
-
-    /// Returns the list of items that this calendar contains
-    async fn get_items(&self) -> Result<HashMap<ItemId, &Item>, Box<dyn Error>> {
-        self.get_items_modified_since(None, None).await
-    }
-
 }
diff --git a/src/calendar/remote_calendar.rs b/src/calendar/remote_calendar.rs
index 883c6c8..e985d60 100644
--- a/src/calendar/remote_calendar.rs
+++ b/src/calendar/remote_calendar.rs
@@ -7,8 +7,9 @@ use async_trait::async_trait;
 use crate::traits::PartialCalendar;
 use crate::calendar::SupportedComponents;
 use crate::calendar::CalendarId;
-use crate::item::ItemId;
 use crate::item::Item;
+use crate::item::ItemId;
+use crate::item::VersionTag;
 use crate::resource::Resource;
 
 static TASKS_BODY: &str = r#"
@@ -51,14 +52,6 @@ impl PartialCalendar for RemoteCalendar {
         self.supported_components
     }
 
-    /// Returns the items that have been last-modified after `since`
-    async fn get_items_modified_since(&self, since: Option<DateTime<Utc>>, _filter: Option<crate::calendar::SearchFilter>)
-        -> Result<HashMap<ItemId, &Item>, Box<dyn Error>>
-    {
-        log::error!("Not implemented");
-        Ok(HashMap::new())
-    }
-
     /// Get the IDs of all current items in this calendar
     async fn get_item_ids(&self) -> Result<HashSet<ItemId>, Box<dyn Error>> {
         let responses = crate::client::sub_request_and_extract_elems(&self.resource, "REPORT", TASKS_BODY.to_string(), "response").await?;
@@ -82,6 +75,11 @@ impl PartialCalendar for RemoteCalendar {
         Ok(item_ids)
     }
 
+    async fn get_item_version_tags(&self) -> Result<HashMap<ItemId, VersionTag>, Box<dyn Error>> {
+        log::error!("Not implemented");
+        Ok(HashMap::new())
+    }
+
     /// Returns a particular item
     async fn get_item_by_id_mut<'a>(&'a mut self, _id: &ItemId) -> Option<&'a mut Item> {
         log::error!("Not implemented");
diff --git a/src/event.rs b/src/event.rs
index ff37d63..829b0ac 100644
--- a/src/event.rs
+++ b/src/event.rs
@@ -4,6 +4,7 @@ use serde::{Deserialize, Serialize};
 use chrono::{Utc, DateTime};
 
 use crate::item::ItemId;
+use crate::item::VersionTag;
 
 /// TODO: implement Event one day.
 /// This crate currently only supports tasks, not calendar events.
@@ -12,6 +13,7 @@ pub struct Event {
     id: ItemId,
     name: String,
     last_modified: DateTime<Utc>,
+    version_tag: VersionTag,
 }
 
 impl Event {
@@ -26,4 +28,8 @@ impl Event {
     pub fn last_modified(&self) -> DateTime<Utc> {
         self.last_modified
     }
+
+    pub fn version_tag(&self) -> &VersionTag {
+        &self.version_tag
+    }
 }
diff --git a/src/item.rs b/src/item.rs
index 86bb923..3f7facb 100644
--- a/src/item.rs
+++ b/src/item.rs
@@ -7,6 +7,8 @@ use url::Url;
 
 use crate::resource::Resource;
 
+
+
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub enum Item {
     Event(crate::event::Event),
@@ -35,6 +37,13 @@ impl Item {
         }
     }
 
+    pub fn version_tag(&self) -> &VersionTag {
+        match self {
+            Item::Event(e) => e.version_tag(),
+            Item::Task(t) => t.version_tag(),
+        }
+    }
+
     pub fn is_event(&self) -> bool {
         match &self {
             Item::Event(_) => true,
@@ -109,3 +118,24 @@ impl Display for ItemId {
         write!(f, "{}", self.content)
     }
 }
+
+
+/// A VersionTag is basically a CalDAV `ctag` or `etag`. Whenever it changes, this means the data has changed.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct VersionTag {
+    tag: String
+}
+
+impl From<String> for VersionTag {
+    fn from(tag: String) -> VersionTag {
+        Self { tag }
+    }
+}
+
+impl VersionTag {
+    /// Generate a random VesionTag. This is only useful in tests
+    pub fn random() -> Self {
+        let random = uuid::Uuid::new_v4().to_hyphenated().to_string();
+        Self { tag: random }
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 4019ea3..d08286f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -15,6 +15,7 @@ pub use calendar::cached_calendar::CachedCalendar;
 mod item;
 pub use item::Item;
 pub use item::ItemId;
+pub use item::VersionTag;
 mod task;
 pub use task::Task;
 mod event;
diff --git a/src/provider.rs b/src/provider.rs
index b0e1cb6..d56e63c 100644
--- a/src/provider.rs
+++ b/src/provider.rs
@@ -7,7 +7,6 @@ use std::marker::PhantomData;
 use chrono::{DateTime, Utc};
 
 use crate::traits::{CalDavSource, CompleteCalendar};
-use crate::traits::SyncSlave;
 use crate::traits::PartialCalendar;
 use crate::Item;
 use crate::item::ItemId;
@@ -16,7 +15,7 @@ use crate::item::ItemId;
 /// A data source that combines two `CalDavSources` (usually a server and a local cache), which is able to sync both sources.
 pub struct Provider<L, T, S, U>
 where
-    L: CalDavSource<T> + SyncSlave,
+    L: CalDavSource<T>,
     T: CompleteCalendar,
     S: CalDavSource<U>,
     U: PartialCalendar + Sync + Send,
@@ -32,7 +31,7 @@ where
 
 impl<L, T, S, U> Provider<L, T, S, U>
 where
-    L: CalDavSource<T> + SyncSlave,
+    L: CalDavSource<T>,
     T: CompleteCalendar,
     S: CalDavSource<U>,
     U: PartialCalendar + Sync + Send,
@@ -51,19 +50,14 @@ where
     pub fn server(&self) -> &S { &self.server }
     /// Returns the data source described as the `local`
     pub fn local(&self)  -> &L { &self.local }
-    /// Returns the last time the `local` source has been synced
-    pub fn last_sync_timestamp(&self) -> Option<DateTime<Utc>> {
-        self.local.get_last_sync()
-    }
 
     /// Performs a synchronisation between `local` and `server`.
     ///
     /// This bidirectional sync applies additions/deletions made on a source to the other source.
     /// In case of conflicts (the same item has been modified on both ends since the last sync, `server` always wins)
     pub async fn sync(&mut self) -> Result<(), Box<dyn Error>> {
-        let last_sync = self.local.get_last_sync();
-        log::info!("Starting a sync. Last sync was at {:?}", last_sync);
-
+        log::info!("Starting a sync.");
+/*
         let cals_server = self.server.get_calendars().await?;
         for (id, cal_server) in cals_server {
             let mut cal_server = cal_server.lock().unwrap();
@@ -77,8 +71,8 @@ where
             };
             let mut cal_local = cal_local.lock().unwrap();
 
-            // Step 1 - "Server always wins", so a delteion from the server must be applied locally, even if it was locally modified.
-            let mut local_dels = match last_sync {
+            // Step 1 - "Server always wins", so a deletion from the server must be applied locally, even if it was locally modified.
+            let mut local_dels = match cal_local.get_last_sync() {
                 None => HashSet::new(),
                 Some(date) => cal_local.get_items_deleted_since(date).await?,
             };
@@ -138,7 +132,7 @@ where
         }
 
         self.local.update_last_sync(None);
-
+*/
         Ok(())
     }
 }
diff --git a/src/task.rs b/src/task.rs
index d56a7ff..f19e9e0 100644
--- a/src/task.rs
+++ b/src/task.rs
@@ -2,6 +2,7 @@ use chrono::{Utc, DateTime};
 use serde::{Deserialize, Serialize};
 
 use crate::item::ItemId;
+use crate::item::VersionTag;
 
 /// A to-do task
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
@@ -11,6 +12,8 @@ pub struct Task {
 
     /// The last modification date of this task
     last_modified: DateTime<Utc>,
+    /// The version tag of this item
+    version_tag: VersionTag,
 
     /// The display name of the task
     name: String,
@@ -20,11 +23,12 @@ pub struct Task {
 
 impl Task {
     /// Create a new Task
-    pub fn new(name: String, id: ItemId, last_modified: DateTime<Utc>) -> Self {
+    pub fn new(name: String, id: ItemId, last_modified: DateTime<Utc>, version_tag: VersionTag) -> Self {
         Self {
             id,
             name,
             last_modified,
+            version_tag,
             completed: false,
         }
     }
@@ -33,6 +37,7 @@ impl Task {
     pub fn name(&self) -> &str      { &self.name        }
     pub fn completed(&self) -> bool { self.completed    }
     pub fn last_modified(&self) -> DateTime<Utc> { self.last_modified }
+    pub fn version_tag(&self) -> &VersionTag     { &self.version_tag  }
 
     fn update_last_modified(&mut self) {
         self.last_modified = Utc::now();
diff --git a/src/traits.rs b/src/traits.rs
index 34fe2d8..ef2216e 100644
--- a/src/traits.rs
+++ b/src/traits.rs
@@ -7,6 +7,7 @@ use chrono::{DateTime, Utc};
 
 use crate::item::Item;
 use crate::item::ItemId;
+use crate::item::VersionTag;
 use crate::calendar::CalendarId;
 
 #[async_trait]
@@ -18,14 +19,6 @@ pub trait CalDavSource<T: PartialCalendar> {
     async fn get_calendar(&self, id: &CalendarId) -> Option<Arc<Mutex<T>>>;
 }
 
-pub trait SyncSlave {
-    /// Returns the last time this source successfully synced from a master source (e.g. from a server)
-    /// (or None in case it has never been synchronized)
-    fn get_last_sync(&self) -> Option<DateTime<Utc>>;
-    /// Update the last sync timestamp to now, or to a custom time in case `timepoint` is `Some`
-    fn update_last_sync(&mut self, timepoint: Option<DateTime<Utc>>);
-}
-
 /// A calendar we have a partial knowledge of.
 ///
 /// Usually, this is a calendar from a remote source, that is synced to a CompleteCalendar
@@ -40,12 +33,8 @@ pub trait PartialCalendar {
     /// Returns the supported kinds of components for this calendar
     fn supported_components(&self) -> crate::calendar::SupportedComponents;
 
-    /// Returns the items that have been last-modified after `since`
-    async fn get_items_modified_since(&self, since: Option<DateTime<Utc>>, filter: Option<crate::calendar::SearchFilter>)
-        -> Result<HashMap<ItemId, &Item>, Box<dyn Error>>;
-
-    /// Get the IDs of all current items in this calendar
-    async fn get_item_ids(&self) -> Result<HashSet<ItemId>, Box<dyn Error>>;
+    /// Get the IDs and the version tags of every item in this calendar
+    async fn get_item_version_tags(&self) -> Result<HashMap<ItemId, VersionTag>, Box<dyn Error>>;
 
     /// Returns a particular item
     async fn get_item_by_id_mut<'a>(&'a mut self, id: &ItemId) -> Option<&'a mut Item>;
@@ -67,6 +56,14 @@ pub trait PartialCalendar {
         self.supported_components().contains(crate::calendar::SupportedComponents::EVENT)
     }
 
+    /// Get the IDs of all current items in this calendar
+    async fn get_item_ids(&self) -> Result<HashSet<ItemId>, Box<dyn Error>> {
+        let items = self.get_item_version_tags().await?;
+        Ok(items.iter()
+            .map(|(id, _tag)| id.clone())
+            .collect())
+    }
+
     /// Finds the IDs of the items that are missing compared to a reference set
     async fn find_deletions_from(&self, reference_set: HashSet<ItemId>) -> Result<HashSet<ItemId>, Box<dyn Error>> {
         let current_items = self.get_item_ids().await?;
@@ -84,6 +81,10 @@ pub trait CompleteCalendar : PartialCalendar {
     /// See also [`PartialCalendar::get_items_deleted_since`]
     async fn get_items_deleted_since(&self, since: DateTime<Utc>) -> Result<HashSet<ItemId>, Box<dyn Error>>;
 
+   /// Returns the items that have been last-modified after `since`
+    async fn get_items_modified_since(&self, since: Option<DateTime<Utc>>, filter: Option<crate::calendar::SearchFilter>)
+        -> Result<HashMap<ItemId, &Item>, Box<dyn Error>>;
+
     /// Returns the list of items that this calendar contains
     async fn get_items(&self) -> Result<HashMap<ItemId, &Item>, Box<dyn Error>>;
 }
diff --git a/tests/caldav_client.rs b/tests/caldav_client.rs
index ec11660..0e5f591 100644
--- a/tests/caldav_client.rs
+++ b/tests/caldav_client.rs
@@ -53,9 +53,9 @@ async fn show_calendars() {
         }
 
         println!("  Most recent changes:");
-        for (_id, task) in cal.get_items_modified_since(None, None).await.unwrap() {
-            my_tasks::utils::print_task(task);
-        }
+        // for (_id, task) in cal.get_item_version_tags(None, None).await.unwrap() {
+        //     my_tasks::utils::print_task(task);
+        // }
     }
 }
 
diff --git a/tests/sync.rs b/tests/sync.rs
index 943d608..6e0601f 100644
--- a/tests/sync.rs
+++ b/tests/sync.rs
@@ -4,11 +4,12 @@ use std::sync::{Arc, Mutex};
 use chrono::{Utc, TimeZone};
 use url::Url;
 
-use my_tasks::traits::{CalDavSource, SyncSlave};
+use my_tasks::traits::CalDavSource;
 use my_tasks::traits::PartialCalendar;
 use my_tasks::cache::Cache;
 use my_tasks::Item;
 use my_tasks::ItemId;
+use my_tasks::VersionTag;
 use my_tasks::Task;
 use my_tasks::calendar::cached_calendar::CachedCalendar;
 use my_tasks::Provider;
@@ -53,23 +54,23 @@ async fn populate_test_provider() -> Provider<Cache, CachedCalendar, Cache, Cach
 
     let cal_id = Url::parse("http://todo.list/cal").unwrap();
 
-    let task_a = Item::Task(Task::new("task A".into(), ItemId::random(), Utc.ymd(2000, 1, 1).and_hms(0, 0, 0)));
-    let task_b = Item::Task(Task::new("task B".into(), ItemId::random(), Utc.ymd(2000, 1, 2).and_hms(0, 0, 0)));
-    let task_c = Item::Task(Task::new("task C".into(), ItemId::random(), Utc.ymd(2000, 1, 3).and_hms(0, 0, 0)));
-    let task_d = Item::Task(Task::new("task D".into(), ItemId::random(), Utc.ymd(2000, 1, 4).and_hms(0, 0, 0)));
-    let task_e = Item::Task(Task::new("task E".into(), ItemId::random(), Utc.ymd(2000, 1, 5).and_hms(0, 0, 0)));
-    let task_f = Item::Task(Task::new("task F".into(), ItemId::random(), Utc.ymd(2000, 1, 6).and_hms(0, 0, 0)));
-    let task_g = Item::Task(Task::new("task G".into(), ItemId::random(), Utc.ymd(2000, 1, 7).and_hms(0, 0, 0)));
-    let task_h = Item::Task(Task::new("task H".into(), ItemId::random(), Utc.ymd(2000, 1, 8).and_hms(0, 0, 0)));
-    let task_i = Item::Task(Task::new("task I".into(), ItemId::random(), Utc.ymd(2000, 1, 9).and_hms(0, 0, 0)));
-    let task_j = Item::Task(Task::new("task J".into(), ItemId::random(), Utc.ymd(2000, 1, 10).and_hms(0, 0, 0)));
-    let task_k = Item::Task(Task::new("task K".into(), ItemId::random(), Utc.ymd(2000, 1, 11).and_hms(0, 0, 0)));
-    let task_l = Item::Task(Task::new("task L".into(), ItemId::random(), Utc.ymd(2000, 1, 12).and_hms(0, 0, 0)));
-    let task_m = Item::Task(Task::new("task M".into(), ItemId::random(), Utc.ymd(2000, 1, 12).and_hms(0, 0, 0)));
+    let task_a = Item::Task(Task::new("task A".into(), ItemId::random(), Utc.ymd(2000, 1, 1).and_hms(0, 0, 0), VersionTag::random()));
+    let task_b = Item::Task(Task::new("task B".into(), ItemId::random(), Utc.ymd(2000, 1, 2).and_hms(0, 0, 0), VersionTag::random()));
+    let task_c = Item::Task(Task::new("task C".into(), ItemId::random(), Utc.ymd(2000, 1, 3).and_hms(0, 0, 0), VersionTag::random()));
+    let task_d = Item::Task(Task::new("task D".into(), ItemId::random(), Utc.ymd(2000, 1, 4).and_hms(0, 0, 0), VersionTag::random()));
+    let task_e = Item::Task(Task::new("task E".into(), ItemId::random(), Utc.ymd(2000, 1, 5).and_hms(0, 0, 0), VersionTag::random()));
+    let task_f = Item::Task(Task::new("task F".into(), ItemId::random(), Utc.ymd(2000, 1, 6).and_hms(0, 0, 0), VersionTag::random()));
+    let task_g = Item::Task(Task::new("task G".into(), ItemId::random(), Utc.ymd(2000, 1, 7).and_hms(0, 0, 0), VersionTag::random()));
+    let task_h = Item::Task(Task::new("task H".into(), ItemId::random(), Utc.ymd(2000, 1, 8).and_hms(0, 0, 0), VersionTag::random()));
+    let task_i = Item::Task(Task::new("task I".into(), ItemId::random(), Utc.ymd(2000, 1, 9).and_hms(0, 0, 0), VersionTag::random()));
+    let task_j = Item::Task(Task::new("task J".into(), ItemId::random(), Utc.ymd(2000, 1, 10).and_hms(0, 0, 0), VersionTag::random()));
+    let task_k = Item::Task(Task::new("task K".into(), ItemId::random(), Utc.ymd(2000, 1, 11).and_hms(0, 0, 0), VersionTag::random()));
+    let task_l = Item::Task(Task::new("task L".into(), ItemId::random(), Utc.ymd(2000, 1, 12).and_hms(0, 0, 0), VersionTag::random()));
+    let task_m = Item::Task(Task::new("task M".into(), ItemId::random(), Utc.ymd(2000, 1, 12).and_hms(0, 0, 0), VersionTag::random()));
 
-    let last_sync = task_m.last_modified();
-    local.update_last_sync(Some(last_sync));
-    assert!(last_sync < Utc::now());
+    // let last_sync = task_m.last_modified();
+    // local.update_last_sync(Some(last_sync));
+    // assert!(last_sync < Utc::now());
 
     let task_b_id = task_b.id().clone();
     let task_c_id = task_c.id().clone();
@@ -129,7 +130,7 @@ async fn populate_test_provider() -> Provider<Cache, CachedCalendar, Cache, Cach
 
     cal_server.delete_item(&task_l_id).await.unwrap();
 
-    let task_n = Item::Task(Task::new("task N (new from server)".into(), ItemId::random(), Utc::now()));
+    let task_n = Item::Task(Task::new("task N (new from server)".into(), ItemId::random(), Utc::now(), VersionTag::random()));
     cal_server.add_item(task_n).await;
 
 
@@ -158,7 +159,7 @@ async fn populate_test_provider() -> Provider<Cache, CachedCalendar, Cache, Cach
     cal_local.delete_item(&task_k_id).await.unwrap();
     cal_local.delete_item(&task_l_id).await.unwrap();
 
-    let task_o = Item::Task(Task::new("task O (new from local)".into(), ItemId::random(), Utc::now()));
+    let task_o = Item::Task(Task::new("task O (new from local)".into(), ItemId::random(), Utc::now(), VersionTag::random()));
     cal_local.add_item(task_o).await;
 
     Provider::new(server, local)