Question

If I use DateTime.Now as part of software licensing authentication to enforce the number of days software can be used, can this be circumvented by simply changing the system clock?

If so is there a better way to get real UTC time?

Answer 1

If I use DateTime.Now as part of software licensing authentication to
enforce the number of days software can be used, can this be
circumvented by simply changing the system clock?

DateTime.Now uses the system clock, so yes it could be circumvented.

If so is there a better way to get accurate system time?

Yes, you could query a time server over the www. But you know, even this could be circumvented because the user can modify his hosts file and replace the address you are using with his own proxy and return whatever time he likes. So it will all depend on how far do you want to get into protecting your software. If you really care about the intellectual property of your software you are better of purchasing a commercial licensing software.