If you are after T-SQL First Day of Previous Month calculation then you are in the right place.
To return first day of previous month using T-SQL we can use the following statement:
SELECT DATEADD(m,-1, Dateadd(d,1-DATEPART(d,getdate()),GETDATE())) AS [First Day of Previous Month]
Below is explanation how the t-sql statement calculates first day of previous month:
1) Our today's date is 20th of Feb. We substract number of days in the month from our date (20) and we get
last day of previous month.
2) We add one day to get first day of current month.
3) We go one month back so first day of the month stays the same and Feb changes to Jan.
The result of this T-SQL statement is first day of previous month.
Hope that helps!