calculate period based on a date (invoice date)

I would like to know if there is a function to calculates the month and the year for you based on a date in this format: "mm/dd/yyyy". The month to derive should be the month of the invoice date -1. For example if the invoice date is 01/10/2012 then the period should show: December, 2011. Another example: when the invoice date is 12/12/2011 then period should be November, 2011, etc...

Any help is most appreciated.
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If you just want the month, you can use:
You can then format it any way you want.
You can also use (in VBA anyway):
Try this:

Format(DateAdd(YourDate,"m", -1), "mmmm, yyyy")
To also show year, you can use the format with "MMMM, yyyy"
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try something like this;

Format(DateAdd("m",-1,[date]),"Mmmm YYYY")

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I got the DateAdd arguments backwards in my comment.

The syntax, as the others have is:

DateAdd(Interval, Number,Date)
thank you very much
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