Summing up time in OpenOffice

Summing hours and minutes (more than 24 hours)

Select the cells to sum; from the menu: Format - Cells...; Category = User-defined; Format code = [HH]:MM. You can now enter hours and minutes, for example 15:15, 25:30. Summing them gives 40:45, displayed correctly.

Summing minutes and seconds (more than 60 minutes)

Use the example for hours and minutes above, but pretend that you are entering minutes and seconds, rather than hours and minutes. So entering 15:15 means 15 minutes 15 seconds.

found on :https://wiki.openoffice.org/wiki/Documentation/How_Tos/Calc:_Date_&_Time_functions

Formular to Convert Hours and minutes into  decimals

=(HOUR(E2)*60+MINUTE(E2))/60

found on http://www.oooforum.org/forum/viewtopic.phtml?t=36320