[Zope] example ?

I would like, within a ZPT to get a global variable called 'now' to the present date and time. I can do this in dtml with <dtml-call "REQUEST.set('now', ZopeTime())"> but how do I do I it in TAL ? This was my guess : <span tal: define="now string: ZopeTime()"> but it wasn't correct :( Kate