UFN_DATE_THISMONTH_FIRSTDAY

        create function dbo.UFN_DATE_THISMONTH_FIRSTDAY(@TODAY datetime, @USELATESTTIME bit)
            returns datetime
            with execute as caller
            as
            begin
                declare @DATE datetime

                set @DATE = @TODAY + 1 - day(@TODAY)

                if @USELATESTTIME = 0
                begin
                    set @DATE = dbo.UFN_DATE_GETEARLIESTTIME(@DATE)
                end
                else
                begin
                    set @DATE = dbo.UFN_DATE_GETLATESTTIME(@DATE)
                end

                return @DATE
            end