how can i calculate the trial period of the license without using system datetime

A simple solution would be to store the dates when the application was installed, first run, and last run. From there, you will have some basis when the system date has changed. For example, if the system date was earlier than the installed date, this should raise some suspicions.

Below is an article about it. Its written in VB.Net but it should be pretty easy to understand.

http://www.dreamincode.net/forums/topic/65721-trial-period/[^]

One solution could be storing the last system date your software was started (in the registry or a hidden file), and compare this date to the current date to prevent people from cheating.

Let's see an example:

- Let's say your application setup stores an "Install date" value in the registry.
- You will also store another value: "Last started". You should initialize it (in your setup) with the same value as "Install date".

Each time your application starts:
- Read the "Last started" value from registry. Compare it to the current system date. If it is less or equal than current system date, then continue normally. If it is greater, it means the user cheated and changed the system date: you can then display an error message or whatever.
- If the initialization should continue normally, update the "Last started" value in the registry with the current system date.
- After that, you can compare the current system date to the "Install date" to know if the trial period is still valid.

You should of course not remove the registry values from the system in your uninstall tool, otherwise the user can re-install your application as many times as he wants and it would reset the registry values...
Moreover, your setup shouldn't overwrite these values (if they already exist, it means the software was already installed)

To protect a little bit more, you can also make the names "Install date", ..., not so obvious. This will prevent user to find them and change them manually.

This is one solution.

There are a few ways to do this;
1. Query a timeserver via sntp.
2. Use a php or asp.net script on your server to get the time.