[Zope] Calculations on a Date Field

[complaw@hal-pc.org]

My users are annoyed with the time shift of postings on a Squishdot site.  The
users are in the U.S. and the server is in the Netherlands (FreeZope.org, a
truly wonderful thing).  The DTML code in Squishdot that displays the date
appears to be a date field in the posting object.  E.g., the code is:

<dtml-var date_posted>

What I want to do is recalculate the date_posted and shift the time back about
five hours.  

I tried:

<dtml-let old_date=date_posted

new_date="old_date - 0.25">
<dtml-var new_date>
</dtml-let>

But the server returned an error regarding the use of the minus sign.  Does
anyone have any hints?

Thanks in advance,

Ron