fix: get_chat_msgs_ex(): Report local midnight in ChatItem::DayMarker

We were reporting the UTC midnight timestamp instead. For UTC-N timezones that means reporting
"yesterday".

Fixes deltachat/deltachat-desktop#5215.

Author: iequidoo

deltachat-ffi/deltachat.h

@@ -1332,12 +1332,14 @@ dc_msg_t*       dc_get_draft                 (dc_context_t* context, uint32_t ch
  * Optionally, some special markers added to the ID array may help to
  * implement virtual lists.
  *
+ * To get the concrete time of the message, use dc_array_get_timestamp().
+ *
  * @memberof dc_context_t
  * @param context The context object as returned from dc_context_new().
  * @param chat_id The chat ID of which the messages IDs should be queried.
  * @param flags If set to DC_GCM_ADDDAYMARKER, the marker DC_MSG_ID_DAYMARKER will
  *     be added before each day (regarding the local timezone). Set this to 0 if you do not want this behaviour.
- *     To get the concrete time of the marker, use dc_array_get_timestamp().
+ *     The day marker timestamp is the midnight one for the corresponding (following) day in the local timezone.
  *     If set to DC_GCM_INFO_ONLY, only system messages will be returned, can be combined with DC_GCM_ADDDAYMARKER.
  * @param marker1before Deprecated, set this to 0.
  * @return Array of message IDs, must be dc_array_unref()'d when no longer used.

deltachat-jsonrpc/src/api.rs

@@ -1227,8 +1227,10 @@ impl CommandApi {
     }
 
     /// Returns all messages of a particular chat.
-    /// If `add_daymarker` is `true`, it will return them as
-    /// `DC_MSG_ID_DAYMARKER`, e.g. [1234, 1237, 9, 1239].
+    ///
+    /// * `add_daymarker` - If `true`, add day markers as `DC_MSG_ID_DAYMARKER` to the result,
+    ///   e.g. [1234, 1237, 9, 1239]. The day marker timestamp is the midnight one for the
+    ///   corresponding (following) day in the local timezone.
     async fn get_message_ids(
         &self,
         account_id: u32,

src/chat.rs

@@ -3323,10 +3323,11 @@ pub async fn get_chat_msgs_ex(
         for (ts, curr_id) in sorted_rows {
             if add_daymarker {
                 let curr_local_timestamp = ts + cnv_to_local;
-                let curr_day = curr_local_timestamp / 86400;
+                let secs_in_day = 86400;
+                let curr_day = curr_local_timestamp / secs_in_day;
                 if curr_day != last_day {
                     ret.push(ChatItem::DayMarker {
-                        timestamp: curr_day * 86400, // Convert day back to Unix timestamp
+                        timestamp: curr_day * secs_in_day - cnv_to_local,
                     });
                     last_day = curr_day;
                 }