I Built an RTC based on TimeLib and NTP synchronization. The RTC displays time using a TFT shield. This RTC operated correctly.
I tried to synchronize several clocks by transmission of time from this RTC using virtualwire library.
I noticed that the time display of the RTC gets frozen after a couple of minutes, although the printout of time on the RTC serial monitor keeps to be updated correctly.
For the TFT display I use MCUFRIEND and ADAFRUIT_GFX libraries.
I would appreciate any idea.