Firstly you need to make sure the relevant columns are in the Time format.
Select columns B, C & D by clicking on the column headers, then right click and select 'Format Cells'. Under the first tab (Number) select 'Time', with the format xx:xx. Then when you enter the time use the format xx:xx with a colon separating the hours from the minutes.
Now use the following formula in column D -
=C2-B2+IF(B2>C2,24)
Everything should now be calculated in hours and minutes. The IF function allows for when your shift goes past midnight i.e. if your end time is a smaller number than your start time.
N.B. this is how it works in Excel, I'm presuming OpenOffice will be the same or similar.
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