why doesnt this work???

[#1]

if(FirstSortedValue(Places_Before_Decimal_Point_In_Meter_Reading,Meter_Reading_Date)<='425'

     or FirstSortedValue(Places_Before_Decimal_Point_In_Meter_Reading,Meter_Reading_Date)>='365','In Period','Out of period') as range,

trying to get earliest value in 365-425 date range, it says 'invalid expression' despite the synttax checker not flagging it up.

[#2]

Maybe remove the single quotes around the numbers.