We should check / verify the system date while activation, if teh date is incorrect, we should give a error message to the user mentioning the same. This will prevent users from carry out the registration after modifying the system daet and then claiming that it was done unknowingly.
I would suggest that we take the Server time/date for Activation rather than depending on the local system date it could only vary by max 24 hours depending on the location in the earth.