EOMONTH in SQL Server: last day of the month

EOMONTH returns the last day of the month of a date. Finance lives on month ends. First of the month is one day after last month’s EOMONTH.

This month so far

DECLARE @today date = CAST(GETDATE() AS date);
DECLARE @start date = DATEADD(day, 1, EOMONTH(@today, -1));

SELECT order_id, order_date, total_amount
FROM Orders
WHERE order_date >= @start
  AND order_date <  DATEADD(day, 1, @today);

EOMONTH(@today, -1) is last day of last month. Add a day, you have this month’s 1st.