How Can I Check To See If the User Set His Clock Back?

Question

In my desktop program, I want to check that to see if the user has set his clock back.

To do so, I compare the timestamp of certain Operating System files to the current computer date. If any are after what the computer thinks is the current date, then he must have set his clock back.

For Windows XP, I have been using such files as:

c:\win386.swp
c:\windows\user.dat
and several others.

Two questions:

1. Is this a decent way to do this, or is there something better?

2. (Assuming the answer to 1 is "decent") What would be some good Windows Vista and Windows 7 files to compare with?

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Conclusion: Priyank said something obvious that had eluded me: Rather than using any system files, the easiest and simplest way might be to just use the timestamp of your program itself. After all, the date it was installed is exactly the date you want your trial to start from.

As it turns out, your answers and my research of other related StackOverflow questions and mentioned articles has led me to decide on a much simpler scheme. My trial users already have to get a key to use the trial. I've got the date registered embedded into the key. I really don't have to check for setting the clock back. Instead I can just seee if the current time is between the date registered and the date registered plus n days.

If they want to go to the trouble of getting around that scheme, then let them. I agree that those people wouldn't pay anyway. Make your program something worthwhile that people enjoy and want to use and most will pay.

Answer 1

An option would be to get the date from a web service or time server. The problem here is that this only works if there is internet connection available, so this would be just a complementary method for a more safe one.

Answer 2

a better way to do that IMHO would be to store the current timestamp in registry the first time your program is ran and then compare that with the current timestamp on next run and update the timestamp if everything is OK.

Answer 3

Windows files will be always have earlier date, the best is to check the creation or modified date of your program with the system date. Or any time your program quit make sure to note the timestamp and compare it with the next time you start your program, and there is always NTP: http://en.wikipedia.org/wiki/Network%5FTime%5FProtocol

Answer 4

Windows provides APIs for notification of system time changes. One e.g. is the sysinfo.ocx activex control provided here.

Answer 5

This is not a direct answer, just to offer some opinion and references. Too long for me to add as comment.

If the purpose is to create a time-limited software for demo/trial use, we need to find a balance between how much you go about implementing it and the tendency for users to 'hack' pass it.

I've often tell people, if your software attracts hackers, it's probably something useful/good. No point protecting a software which no one finds useful.

Anyway, the philosophical talk aside, below are a couple good reads about this topic:

How to Implement time-limited trial on Joel's Forum
Time limited trial and Windows Certification on SO
Need Advice on Implementing a Time-limited Trial on SO