Make build deprecation more resilient to clock skew.

app/src/main/java/org/thoughtcrime/securesms/keyvalue/MiscellaneousValues.kt

@@ -202,6 +202,12 @@ internal class MiscellaneousValues internal constructor(store: KeyValueStore) :
    */
   val lastKnownServerTimeOffset by longValue(SERVER_TIME_OFFSET, 0)
 
+  /**
+   * An estimate of the server time, based on the last-known server time offset.
+   */
+  val estimatedServerTime: Long
+    get() = System.currentTimeMillis() - lastKnownServerTimeOffset
+
   /**
    * The last time (using our local clock) we updated the server time offset returned by [.getLastKnownServerTimeOffset]}.
    */