InfoPath Dev

I'm using InfoPath 2007.  How can I ensure a user enters the time portion of a DateTime field?  The Date Picker will fill in the date portion, but users are skipping the time portion.

It doesn't look like I can build an input mask, per se.  My thought was to try to use XQuery; something like

if hours-from-dateTime(myDateTimeField) = 0 and if minutes-from-dateTime(myDateTimeField) = 0 then ...

but InfoPath tells me that "'hours-from-dateTime' is not a valid XSLT or XPath function". 

Thanks in advance for any help.

Christian Bahnsen