I want to select a date from a calender in my tool, but I think that it's not possible in the xml.
Correct, date inputs are not currently implemented in Galaxy parameters.
Is this possible? With javascript? Or what?
It's possible if you want to extend Galaxy a bit. I suggest (a) adding a DateParameter to Galaxy (probably extending TextToolParameter; see lib/galaxy/tools/parameters/basic.py) and then use the jQueryUI's date picker:
Good luck,
J.