Better to sync with the server clock when the app starts. It's not a good idea to repeatedly update a time field in the database (or to repeatedly poll for a new time value). It's an unnecessary load on the dbserver (requires an app to run continuously on the server to do the update) and if it is only updating once a minute than for 59 seconds out of each minute the time will be incorrect. If the app is only running for an hour or two then the two clocks shouldn't get too out of sync. If that's a concern then the app could resync the clocks (for example) once an hour.