Sum a DATETIME field

Question

Just noticed it isn't straight forward to SUM a DATETIME fields.
I’ve used the following code to get the time, how do I convert the output back to a DATETIME field?
declare @T table
total_hours_worked datetime
insert into @T values ('01:00:00')
insert into @T values ('01:30:00')
insert into @T values ('03:35:00')
select *
from @T
select SUM((DATEPART(hh,total_hours_worked)*60)+DATEPART(mi,total_hours_worked)+(DATEPART(ss,total_hours_worked)/(60.0)))/60.0 as TotalHours from @T
Thanks,

Answer 1

I should have added the disclaimer that my solution would require total_hours_worked to be stored as either a float or a varchar to be used with the function I described above.   The completed functions would be similar to 
CREATE FUNCTION dbo.TimeSpan_CreateFromString
@pFormattedTimeSpan varchar(128) = null
RETURNS float(53)
with returns null on null input AS
BEGIN
declare @DayPortion int
,@HourPortion int
,@MinutePortion int
,@SecondPortion decimal(6,4)
declare @l int
declare @s2 varchar(128)
DECLARE @Result float(53)
select @s2=LTRIM(rtrim(@pFormattedTimeSpan))
set @l = LEN(@s2)
declare @i int
if CHARINDEX('.',@s2,1) < CHARINDEX(':',@s2,1)
begin
set @i = CHARINDEX('.',@s2,1)
select @DayPortion =
case CHARINDEX('.',@s2)
when 0 then 0
else
case ISNUMERIC(SUBSTRING(@s2,1,CHARINDEX('.',@s2)-1))
when 1 then CAST(SUBSTRING(@s2,1,CHARINDEX('.',@s2)-1) as int)
else 0
end
end;
set @s2=ltrim(rtrim(SUBSTRING(@s2,@i+1,@l-(@i))))
end
else
begin
set @DayPortion=0
end
-- ** debug: select @i, @l, @s2
--select CHARINDEX(':',@s2,1) [c1], CHARINDEX(':',@s2,CHARINDEX(':',@s2,1)+1) [c2], CHARINDEX(':',@s2,(CHARINDEX(':',@s2,CHARINDEX(':',@s2,1)+1))+1) [c3]
set @i = CHARINDEX(':',@s2,1)
select @HourPortion =
case CHARINDEX(':',@s2,1)
when 0 then 0
else
case ISNUMERIC(SUBSTRING(@s2,1,CHARINDEX(':',@s2,1)-1))
when 1 then CAST(SUBSTRING(@s2,1,CHARINDEX(':',@s2,1)-1) AS int)
else null
end
end
set @s2=ltrim(rtrim(SUBSTRING(@s2,@i+1,@l-(@i))))
-- ** debug: select @i, @l, @s2
set @i = CHARINDEX(':',@s2,1)
select @MinutePortion =
case CHARINDEX(':',@s2,1)
when 0 then 0
else
case ISNUMERIC(SUBSTRING(@s2,1,CHARINDEX(':',@s2,1)-1))
when 1 then CAST(SUBSTRING(@s2,1,CHARINDEX(':',@s2,1)-1) AS int)
else null
end
end
set @s2=ltrim(rtrim(SUBSTRING(@s2,@i+1,@l-(@i))))
-- ** debug: select @i, @l, @s2
set @i = CHARINDEX(':',@s2,1)
select @SecondPortion =
case ISNUMERIC(@s2)
when 0 then null
else CAST(@s2 as decimal(6,4))
end
SELECT @Result = dbo.TimeSpan_CreateFromParts(@DayPortion,@HourPortion,@MinutePortion,@SecondPortion);
RETURN @Result
END
GO
I changed the function names to make them more consistent.