Each entry in the Paymo has two time properties - 'start' time and 'end' time.
The time is always passed around in MySQL 'datetime' format (for both getting time from and setting time via the API), which looks like this:
2008-02-28 15:30:00
Which specifies February 28th 2008 at 3:30pm.
The time should always be in the timezone of the authenticated user, which is to say, don't perform any conversion on it.
Other objects may require only the date part without the time as the 'Due Date' for tasks.
Date is passed in MySQL 'date' format like this:
2009-03-19
Which specifies March 19th 2009.