Define customized date format

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")