Interesting, I took a very different approach when I wrote this similar function years ago.


CREATE FUNCTION dbo.DateTime2Precision (@dt datetime2(7), @precision varchar(7))
    returns datetime2
AS
-- written BY Bill Talada
BEGIN
DECLARE
    @filler varchar(27),
    @length int,
    @fixed datetime2

SET @filler = 'cc00-01-01 00:00:00.0000000'
            --'ccyy-mm-dd hh:mm:ss.mmmmmmm'

SET @length =
    CASE @precision
    WHEN 'century' THEN 2
    WHEN 'year' THEN 4
    WHEN 'month' THEN 7
    WHEN 'day' THEN 10
    WHEN 'hour' THEN 13
    WHEN 'minute' THEN 16
    WHEN 'second' THEN 19
    ELSE 0
    END

IF @length = 0
    SET @fixed = @dt
ELSE
    SET @fixed = cast(LEFT(CONVERT(varchar(27), @dt, 121),@length) + RIGHT(@filler,27-@length) AS datetime2)

    RETURN @fixed
END
go

/*
DECLARE @dt datetime2;
SET @dt = SYSDATETIME();
SELECT
    dbo.DateTime2Precision(@dt, 'century'),
    dbo.DateTime2Precision(@dt, 'year'),
    dbo.DateTime2Precision(@dt, 'month'),
    dbo.DateTime2Precision(@dt, 'day'),
    dbo.DateTime2Precision(@dt, 'hour'),
    dbo.DateTime2Precision(@dt, 'minute'),
    dbo.DateTime2Precision(@dt, 'second'),
    dbo.DateTime2Precision(@dt, '')
;
*/