This is returning the last day of the month from the field {Orders.Order Date}.
For example 9-13-08 becomes 9-30-08.
The first part
DateAdd ('m' , 1 , {Orders.Order Date}
adds one month to the field (e.g. 9-13-08 becomes 10-13-08).
The second part
- day({Orders.Order Date}) + 1)
subtracts the total number of days from the month and then adds one day making it the first day of the new month (e.g. 10-13-08 becomes 10-1-08).
The last part
subtracts one day from the first day of this new date making it the last day of the previous month (e.g. 10-1-08 beomes 9-30-08).
The DateAdd function deals with off number months to handle things so as to not create invalid dates. For example adding one month to 1-31-08 will return a value of 2-29-08 instead of 2-31-08 which is invalid. It also handles leap years.
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