I'm currently working on adding date and time pickers to XUL. There have been a number of attempts at this (there are actually at least three calendar widgets in the Mozilla source) so I started by building on top of these. Now, however, the code doesn't look anywhere near the same, but the widgets are more complete I think.
The date picker comes in several varieties, for direct entry using textboxes, a grid and a combination entry and popup grid. I decided not to have a popup time picker and to just have the entry type. The image also includes a number entry box.
On the unrelated to Mozilla side, if you happen to be in the Toronto area on Monday July 31 and have nothing to do, yet want to be entertained with wild and crazy comedy, you're in luck. You would want to attend the second showing of Da Tory Code, co-written by me, Neil Deakin. The first showing has received rave reviews from real life members of parliament, so you know you won't be disappointed. Also, it's free, so what more could you ask for?