I'm trying to update a date field Last_QCCP_Call__c using a field update with the following:

I'm getting the following error:  Error: Formula result is data type (Boolean), incompatible with expected data type (Date).

IF(today() <= DATE(2011,06,01), Last_QCCP_Call__c=DATE(2011,03,01),
IF(today() <= DATE(2011,09,01), Last_QCCP_Call__c=DATE(2011,06,01),
IF(today() <= DATE(2011,12,01), Last_QCCP_Call__c=DATE(2011,09,01),
IF(today() <= DATE(2012,03,01), Last_QCCP_Call__c=DATE(2011,12,01), null
)
)
)
)