Getting an accounting date is not valid error

Hi all,

we have opened April in Purchasing, payables and GL and the encumbrance year is also open  however when we go in to approve a po we are getting the following error

Line 1, schedule 1  accounting date is not in an open encumbrance period.

Also when we try to create a new requisition in april 2012 (all periods open)  we get an error;

invalid date.

Please help.

Thanks

Samara