TOPIC:

I want that the error message should be thrown when user try to create two event for same date and time.
Please suggest me any solution.

Even with different locations?
Even with overlapping time intervals! But this is not that simple: What if one of the events has a start time, but no endtime? That is rather common...

You see, there are a some question, how to handle such events.

There is no simple solution. Someone must create an extension that checks the database and calculates the time intervals.
Due to the lack of developers this probably will not be possible from the currently small team, specially because your request is not very common and would have rather low priority.
But feel free to engage yourself!