code: def getInterval(self):
          
          items = (
              ("", _(u"Not set")),
              ("1", _(u"daily")),
              ("7", _(u"weekly")),
              ("30", _(u"monthly")),
              ("90", _(u"quarterly")),
              ("180", _(u"biannually")),
              ("365", _(u"yearly")),
          )
          return DisplayList(items)
docs: Vocabulary of date intervals to calculate the "To" field date based
          from the "From" field date.