Encrypt a date for use in time limited demo

My app creates a rigistry entry of the date when it is installed, which is then used to determine when the demo expires...It is working fine but I would like to encrypt the date somehow...is there a relative easy way to do this, as different machines will have different date formats?

I am using Visual Studio 2005(VB.Net)

[335 byte] By [ChristieMyburgh] at [2007-12-24]