CREATE FUNCTION dbo.DATE_DIFF(@UNITE VARCHAR(16), @DT_BEGIN DATETIME2, @DT_END DATETIME2)
RETURNS BIGINT
AS
BEGIN

SET @UNITE = LOWER(@UNITE)

-- trivial date's differences
IF @UNITE = 'year'    RETURN CAST(DATEDIFF(year,  @DT_BEGIN, @DT_END) AS BIGINT);
IF @UNITE = 'quarter' RETURN CAST(DATEDIFF(year,  @DT_BEGIN, @DT_END) AS BIGINT);
IF @UNITE = 'month'   RETURN CAST(DATEDIFF(month, @DT_BEGIN, @DT_END) AS BIGINT);
IF @UNITE = 'week'    RETURN CAST(DATEDIFF(year,  @DT_BEGIN, @DT_END) AS BIGINT);
IF @UNITE = 'day'     RETURN CAST(DATEDIFF(day,   @DT_BEGIN, @DT_END) AS BIGINT);

-- for smallest pieces of time use a BIGINT to calculate DIFF and begin to store days differences
DECLARE @DATEDIFF BIGINT = DATEDIFF(day, @DT_BEGIN, @DT_END);
DECLARE @h1  BIGINT = DATEPART(hour, @DT_BEGIN),        @h2  BIGINT = DATEPART(hour, @DT_END),
        @m1  BIGINT = DATEPART(minute, @DT_BEGIN),      @m2  BIGINT = DATEPART(minute, @DT_END),
        @s1  BIGINT = DATEPART(second, @DT_BEGIN),      @s2  BIGINT = DATEPART(second, @DT_END),
        @ms1 BIGINT = DATEPART(millisecond, @DT_BEGIN), @ms2 BIGINT = DATEPART(millisecond, @DT_END),
        @ns1 BIGINT = DATEPART(nanosecond, @DT_BEGIN),  @ns2 BIGINT = DATEPART(nanosecond, @DT_END),
        @µs1 BIGINT = DATEPART(microsecond, @DT_BEGIN), @µs2 BIGINT = DATEPART(microsecond, @DT_END);

-- for hour, take @DATEDIFF (days) * 24 + differences between hours
IF @UNITE = 'hour'        RETURN @DATEDIFF * 24 + @h2 - @h1;
-- for minute, take @DATEDIFF (days) * 24 * 60  + difference between hours * 60 + difference between minutes
IF @UNITE = 'minute'      RETURN @DATEDIFF * 1440 + 60 * (@h2 - @h1) + @m2 - @m1;
-- and so on...
IF @UNITE = 'second'      RETURN @DATEDIFF * 86400 + 3600 * (@h2 - @h1) + 60 * (@m2 - @m1) + @s2 - @s1;
IF @UNITE = 'millisecond' RETURN @DATEDIFF * 86400000 + 3600000 * (@h2 - @h1) + 60000 * (@m2 - @m1) + 1000 * (@s2 - @s1) + @ns1 - @ns2;
IF @UNITE = 'microsecond' RETURN @DATEDIFF * 86400000000 + 3600000000 * (@h2 - @h1) + 60000000 * (@m2 - @m1) + 1000000 * (@s2 - @s1) + 1000 * (@ns1 - @ns2) + @µs2 - @µs1;

RETURN @DATEDIFF;

END;
GO

SELECT DATEDIFF_BIG (microsecond, '0001-01-01', '9999-12-31')

SELECT dbo.DATE_DIFF('microsecond', '0001-01-01', '9999-12-31')

DATEDIFF_BIG