Difference in minutes from datetime - sql

Question

I'm trying to obtain the total amount of time difference from two timestamp columns (datetime)
I currently have a Table 1 setup like the following:
Time_Line_Down => datetime
Time_Line_Ran => datetime
Total_Downtime => Computed column with formula:
(case when [Time_Line_Down] IS NULL then NULL else CONVERT([varchar],case when [Time_Line_Ran] IS NULL then NULL else [Time_Line_Ran]-[Time_Line_Down] end,(108)) end)
Every time some conditions occur, I am copying those three columns (I have more columns but the problem is on this ones) into another Table 2 originally setup like the following:
Time_Line_Down => datetime
Time_Line_Ran => datetime
Total_Downtime => datetime
I then use an excel spreadsheet to "Get External Data" from SQL Server and use a pivot table to work with the data.
Example
Time_Line_Down = 2015-02-20 12:32:40.000
Time_Line_Ran = 2015-02-20 12:34:40.000
Total_Downtime = 1900-01-01 00:02:00.000
Desired Output
I want the pivot table to be able to give me a Grand Total of downtime from all rows in that table
Let's say it was forty five hours, fifty minutes and thirty seconds of accumulated downtime it should read like (45:50:30)
The problem:
Even if I format the Total_Downtime column in the excel pivot table as h:mm:ss to read like this:
Total_Downtime = 0:02:00
As rows accumulate and the Grand Total is calculated the "Date" part of the timestamp is messing the result is the total exceeds 24 hours
What I have tried
I changed the data type format of column Total_Downtime in Table 2 to time(0) so that it won't send the "Date" part, only the "Time" part of the timestamp, it is working and reads out 00:02:00
But now all the values in my pivot table on excel for that column are 0:00:00 no matter what value is actually in the SQL table.
Any suggestions?

Answer 1

You can use the Excel time format [h]:mm:ss which can go beyond 24 hours.
Alternatively, you can use the SQL function DATEDIFF to get the total downtime in seconds, and then convert that to however you need to display it in Excel, e.g.
case when [Time_Line_Down] IS NULL then NULL else case when [Time_Line_Ran] IS NULL then NULL else datediff(ss, Time_Line_Ran, Time_Line_Down) end end
I don't think you need the CASE statements here, you can just use
datediff(ss, Time_Line_Ran, Time_Line_Down)

Answer 2

Thank you all for your help,
I went ahead an tried the function DATEDIFF as suggested, I changed Table 1 computed column formula and Table 2 Total_Downtime column data type to int. Once imported into excel this numeric value needed some extra calculations.
In principle is the best answer and should work for anyone trying to calculate the difference from two timestamps, as mentioned before, is pretty straight forward.
But in my situation I needed to maintain two things:
1) The format 00:00:00 for the column Total_Downtime in Table 1, which changed to an integer value when using DATEDIFF
2) The pivot table Total_Downtime column format [h]:mm:ss (suggested by TobyLL) in excel, which required several calculations to convert from seconds
Solution
After learning that every time I copied from Table 1 to Table 2 the computed value (e.g. 00:02:00) changed to 1900-01-01 00:02:00.000 and that when imported to Excel it equaled to 1.001388889, I decided to force the "Date" part of the time stamp to be 1899-12-31 so that Excel would only calculate the Grand total in the pivot table with the "Time" (decimal) part.