Use mid function to find year, month and day:
mid("","","") string, offset, n
year:
mid(format(GetValue("DATE"),"Date","dd.mm.yyyy", ),"6","4")
month:
mid(format(GetValue("DATE"),"Date","dd.mm.yyyy", ),"3","2")
days:
mid(format(GetValue("DATE"),"Date","dd.mm.yyyy", ),"0","2")
Combine all above in rule:
mid(format(GetValue("DATE"),"Date","dd.mm.yyyy", ),"6","4")+"-"+
mid(format(GetValue("DATE"),"Date","dd.mm.yyyy", ),"3","2")+"-"+
mid(format(GetValue("DATE"),"Date","dd.mm.yyyy", ),"0","2")