Date and time - insert into website

I am making my own forum. I have only limited knowledge of dates and times. When someone makes a post I want to detect the time of the post and the date or maybe even the date time.

How would I write this in PHP. If I store this in a date and time within a database are there any problems with that

 

Use the time() function.

 

I always store it in the database using a bigint(12) , and the function time()

and when extracting using date("Y-m-d h:i",$time)

But this is just me.. This way i can delete "old entries" and such easily..