Hi Celeste,
All dates are calculated in milliseconds from 01 January, 1970 00:00:00 Universal Time (UTC) with a day containing 86,400,000 milliseconds.
Which is the official definition although not particularly helpful.
For pretty detailed information on how to manipulate these beasties try:
http://www.w3schools.com/jsref/jsref_obj_date.asp
Although W3Schools has been ridiculed in the past for some rather schoolboy errors, I have found that latterly they are fairly reliable and this page in particular seems to be pretty good.