You have to understand how XL sees time.

To XL a day is 1, an hour 1/24, a minute 1/1440 etc

So if you enter =1/1440 in a cell and format as , say h:mm ( using custom format) XL shows 00:01

Formatting is ONLY the way XL prsents things to the user, it is not the underlying value ( when time and date are concerned). So formatting a number to time/ date will not do what you expect.

In you example, in cell L12 enter =((H12/J2)*60)/1440 and custom format as h:mm

XL will show you 00:12 as expected ( the background value is 1/120)

Perhaps read http://www.cpearson.com/excel/datetime.htm to understand better how it works

ANother tip: when working with date/time DO NOT realign cell contents manually before you're satisfied all is OK.
Numbers always right align automatically and text aligns left. Sometimes one sees 10:00 that could be text( usually imported files). This way you can easily spot what goes wrong

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