and easy one is time()
http://us2.php.net/manual/en/function.time.php
* DateTime::add — Adds an amount of days, months, years, hours, minutes and seconds to a DateTime object
* DateTime::__construct — Returns new DateTime object
* DateTime::createFromFormat — Returns new DateTime object formatted according to the specified format
* DateTime::diff — Returns the difference between two DateTime objects
* DateTime::format — Returns date formatted according to given format
* DateTime::getLastErrors — Returns the warnings and errors
* DateTime::getOffset — Returns the daylight saving time offset
* DateTime::getTimestamp — Gets the Unix timestamp
* DateTime::getTimezone — Return time zone relative to given DateTime
* DateTime::modify — Alters the timestamp
* DateTime::__set_state — The __set_state handler
* DateTime::setDate — Sets the date
* DateTime::setISODate — Sets the ISO date
* DateTime::setTime — Sets the time
* DateTime::setTimestamp — Sets the date and time based on an Unix timestamp
* DateTime::setTimezone — Sets the time zone for the DateTime object
* DateTime::sub — Subtracts an amount of days, months, years, hours, minutes and seconds from a DateTime object
* DateTime::__wakeup — The __wakeup handler
