[Zope] Property Problem (type converting)

[Sven Hohage]

I like the property of a folder being a list, because new entries should
be automatically updated(strings with the current month-name).
I've tried tokens but it's not working correctly.
<dtml- if "not MonthNow in oldTime">
    <dtml-call "oldTime.append(MonthNow)">
    <dtml-call "entries.manage_changeProperties({'months':
oldTime})">//months is the token-property
</dtml- if>
The problem is that every new entry is a single list.I've tried
type-converting but it's not working for me.
             Many thanks for any hint.