If there is a separate server and there is no real way the server's real time clock can be tampered, than follow Mehdi Gholam's advice and use the server's clock. Actually this is the best practice.
But you haven't dtold us that the database is residig on a differenct machine. In this situation the above statement is useless. You can however use external time sources using NTP and SNTP protocol, to validate local clock (
SNTP Client in C#[
^]).
Still, if your system is isolated, this won't work either. There are very complicated methos you could implement to detect local time tampering, but I don't know if you really need them.