That is what I'm looking for. There are clocks that synchronizes to radio signals (for example DCF77 in Germany), but my problem is that the signal is too weak

That is what I'm looking for. There are clocks that synchronizes to radio signals (for example DCF77 in Germany), but my problem is that the signal is too weak inside my house, and there is no information given when the clock has lost the synchronization. Maybe I look into this and build my own :)