This little snippet is used in a filter and automatically looks at the previous calendar month.
cast([Sales Team].[Client Contacts].[Start Date],date) between _first_of_Month(_add_months(CURRENT_DATE,-1)) and _last_of_month(_add_months(CURRENT_DATE,-1))
If the date is left as a timestamp then the last of the month is 2013-04-30 00:00 which means that a client contact that occurs at 9am would not be picked up. Casting it as a date ensures that it gets picked up as expected.
Another option is to add 1 day onto the _last_of_month, but that’s a hack.
The _add_months function can accept a negative number, which turns _add_months into _substract_months.