The Calendar of Harptos is the calendar used almost all over Faerûn. It was created by the wizard Harptos although most people associate the word ‘Harptos’ with the calendar. It is split into twelve months, each lasting three tendays (or thirty days).
Different regions use different starting points when numbering the years. Most dates will be given in Dale Reckoning (DR). Dalereckoning is taken from the Year of Sunrise, when the Standing Stone was raised by the elves of Cormanthyr and the human Dalesfolk. Since this time, humans were permitted by the Elven Court to settle in the more open regions of the forests. In some texts, primarily those which do not have direct ties to Dales history, Dalereckoning is called Freeman’s Reckoning (FR). The calendar is widely used in Faerûn but has not spread beyond its shores.
Wizards of the Coast also provides a javascript calendar: http://www.wizards.com/default.asp?x=dnd/frx/20050525x