OData filter for a date greater than -2Y

I'm struggling with an OData connection which is trying to retrieve purchase lines of type Order - works fine - but with the additional filter of Only Orders from the last 2 years (to reduce traffic and improve refresh time) as below however I get a Bad Request. This is equivalent to filtering in Navision for the Order Date >-2Y

OData/PurchaseLinesList?$filter=Document_Type eq 'Order' and Order_Date gt '-2y'

Any ideas welcome?