How to formulate on specific dates

Lets say I have 5 invoices each having a total of 100 4 of the invoices are in the month of Jan and 1 in the month of Feb I would like to sum ONLY the 4 invoices of Jan thereby giving me a total of 400

I would expect to write a formula such as if invoice.date in period 01.01.2015 to 31.01.2015 then sum invoice.total

however when I try this, the result is 500 as it's totalling all invoices and note only the 4 in the period.