We can generate number of seconds measured since the Unix Epoch (January 1 1970 00:00:00 GMT) for any given Greenwich Mean Time GMT date and time by using gmmktime function of PHP. Here is the general syntax of mktime() function of PHP.
gmmktime (hour, minute, second, month, day, year)
Output is timestamp
This function is same as mktime() except the passed parameters represents a GMT date.