Igor
To lose the time and date settings after a reload is not uncommon, based on the type of router. Some routers (especially higher end of the product line) have a clock in hardware and will retain time and date over a reload. Other routers (especially the lower end of the product line) do not have hardware clocks and maintain time in software, but these will lose the time setting at reload.
This is why it is frequently good to run NTP on your routers which will learn time from an NTP source. Perhaps there is a reliable NTP source in your network (maybe some other router which does have a hardware clock) or you might run NTP and querry time from one of several time sources on the Internet.
HTH
Rick
HTH
Rick