Если вы хотите отформатировать его как строку, используйте CONVERT. Если вы хотите фактически усечь дробную часть даты и времени, используйте DATEADD, чтобы вычесть ее.
declare @now datetime;
declare @floored datetime;
set @now=GETDATE();
-- get rid of the factional part by subtracting it.
-- datetime has worse than 1 msec of precision so this is exact.
-- datetime2 has 100 nsec precision; will need to use ns then.
set @floored = DATEADD(MS,-DATEPART(ms,@now),@now);
-- output as value and as string.
-- see documentation on date and time styles. I like ODBC (120/121).
select
@now as GetDateTime,
@floored as GetDateFloored,
CONVERT(varchar,@now,120) as GetDateString,
CONVERT(varchar,@floored,120) as GetDateFlooredAsString