I would use the following example SQL statement:
[tt]
SELECT Format(YourDateField,'m/yyyy'), SUM(AFieldToSummarize)
FROM YourTable
GROUP BY Format(YourDateField,'m/yyyy')
[/tt]
or if you prefer using fields that aren't overloaded...
[tt]
SELECT Month(YourDateField), Year(YourDateField), SUM(AFieldToSummarize)
FROM YourTable
GROUP BY Month(YourDateField), Year(YourDateField)
[/tt]