Can you give more information about what it's not liking about your dates/times? For example, does it recognize the field as a Date or DateTime field (based on the symbol next to the field name), or is it reading it as a text field?
If the latter, there could be a few reasons which we can't really tell unless we see what type of format your date/time fields are in. There could be odd spaces, it could be formatted in an unrecognizable way (like '201202'), or any number of issues - all of which are solvable.
But, barring that, this is the simplest fix:
Create a new calculated field, (right click on the variable, choose Create Calculated Field). Say it's a Date field called "Datefield", then write:
Or use DATETIME if a DateTime field.
Hope this helps! But if this doesn't work, attach a few sample fields or a sample workbook and we can see what the specific issue is and go from there.