Calc: sum up dates in from different columns in a third column?


I have an issue with Calc.
I’m using libre Office Version: , english .

I have a column containing full dates (including time), to which I need to add 9 hours (in another column).

The first column is formated as DD-MM-JJJJ UU:MM:SS

Second column is formated as UU:MM

I now want to add these columns

In a third column, I am using formula =Column1+Column2 (Or “=O2+N2” , next row “O3+N3” , etc.)
The formulas get adapted coorectly according to each row.

Still, 9 hours get added, but also another day gets added! The final date is not 30-04 anymore, but 1-05, 02-05 etc. even though this cannot be explained by 9 hour time difference.

How come? What can I do to prevent this (just 9 hours need to be added to the original date…)? Does anyone know?

Kind regards,


Are you sure that the cell showing 9:00 actually contains the numeric value 9/24 = 3/8 = 0,375?

You should be aware of the fact that “Time Of Day” and a “Duration” are very different things. Never format a duration with the code “HH:MM” or “HH:MM:SS” or similar. Always use “[HH]” or “[H]” for the first part to make sure that nothing most relavant (a full day, e.g. caused by an added value 1) is suppressed for the display.

If this was not your problem, please attach an example document demonstrating the error.

See also attached. ask50954AddDurationToDateTime001.ods

If what @Lupp comment doesn’t solve the problem, I remember some issues about dates with some time zones.

problem solved! Thank you, this was the exact answer to my problem! It was thus indeed a formatting error, and not a bug. Again, thanks a lot!

@Nils-EuroClix Glad to have helped. You might mark the answer as correct then.

Not enough points to vote, yet, sorry :wink:

Don’t worry!