接受php中的国际日期时间输入

I'm currently adding support for internationalisation to a system written in PHP. All dates are now stored as UTC, and displayed according to individual user localisation preferences.

However when a user inputs a date time (such as to specify a certain time window), the date time they input gets interpreted as a UTC datetime, not their local datetime. For the internationalisation to be complete the system needs to assume that a datetime entered by the user refers to their local time.

How do I convert a date string (ie 'YYYY-MM-DD HH:MM') into a unix timestamp for the correct localisation?

• Further Clarification - All dates in database = UTC Timestamps All HTML Pages display users local time (as defined in their settings) HTML form has date that defaults to current local time PHP must treat that date time as local not UTC PHP must convert this local date timestring into UTC timestamp

Assuming you know the timezone of the user, which he presumably chose in the preferences somewhere:

$timezone = new DateTimeZone($usersTimezone);
$datetime = new DateTime('2012-01-18 20:00:00', $timezone);
echo $datetime->getTimestamp();

This requires a recent version of PHP with DateTime.