I want to know how the evaluation version of any professional software
works. For example if there is a evaluation software which is supposed
to expire after 120 days, how does the software keeps track of whether
120 days has passed or not. Also a professional software does not get
affected even if you change the system date time or even the DOS time.
I myself 'am a software professional and i need a technical answer for
the same. If there 's some specific code / API's that needs to be used
i need to know the detailed code.
In this case how will the software calculate the elapsed time and
check whether 120 days have passed or not.

There will be no APIs

Nowadays 'call back to momma' is most likely
- although one could count a 'day' as any time the clock appears to go backwards
- like latest recorded time is : 16th April 16:00  and now it is 16th April 9:00

For ease people stick stuff in the Registry, but any App that creates
data files can (and is wise to) stick it in there.

People can get round these things pretty easily by using VMWARE or by
re-imaging their system.

If you really want to find out about the tricks then look at the
Shareware NGs, you'll find that they are pretty cagey, but there are
things like the ASP that you can join - where they open up.

Personally I don't consider time limiting a good idea
- dead Apps don't sell your product

Tieing Apps to hardware is annoying.
Smart people are suspicious of surrepticious TC/IP connections