Handy few SQL snippets. I find that I need to look this up often (from my own script collection), so I might as well post it and I can just search my blog whenever I need it

DECLARE @tdy DATETIME
SET @tdy = GETDATE()
DECLARE @FirstDay1MonthAgo DATETIME
DECLARE @LastDay1MonthAgo DATETIME
DECLARE @FirstDay2MonthAgo DATETIME
DECLARE @LastDay2MonthAgo DATETIME
DECLARE @FirstDay3MonthAgo DATETIME
DECLARE @LastDay3MonthAgo DATETIME
DECLARE @FirstDayCurrentMonth DATETIME
DECLARE @LastDayCurrentMonth DATETIME
SELECT
    @FirstDayCurrentMonth = DATEADD(MONTH, DATEDIFF(MONTH, 0, @tdy), 0),
    @LastDayCurrentMonth = DATEADD(DAY, -1, DATEADD(MONTH, DATEDIFF(MONTH, 0, @tdy) + 1, 0)),
    @FirstDay1MonthAgo = DATEADD(MONTH, DATEDIFF(MONTH, 0, @tdy) - 1, 0),
    @LastDay1MonthAgo = DATEADD(DAY, -1, DATEADD(MONTH, DATEDIFF(MONTH, 0, @tdy), 0)),
    @FirstDay2MonthAgo = DATEADD(MONTH, DATEDIFF(MONTH, 0, @tdy) - 2, 0),
    @LastDay2MonthAgo = DATEADD(DAY, -1, DATEADD(MONTH, DATEDIFF(MONTH, 0, @tdy) - 1, 0)),
    @FirstDay3MonthAgo = DATEADD(MONTH, DATEDIFF(MONTH, 0, CURRENT_TIMESTAMP) - 3, 0),
    @LastDay3MonthAgo = DATEADD(DAY, -1, DATEADD(MONTH, DATEDIFF(MONTH, 0, @tdy) - 2, 0))