[Zope] Property Problem (type converting)

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.