30:00:00 is 30 hours, not 30 minutes. If you want 30 minutes it is 0:30:00.
If you have a large number of cells to convert, put 60 in a cell and copy
it, then select the range of cells you want to convert, and use Edit/ Paste
Special/ Divide.

Unfortunately if you type in 30:00, Excel will assume that you mean 30h 00m.
If you mean 30m 00s, you need to type in either 0:30:0 or 30:0.0.
--
David Biddulph

"P Boric" wrote in message
...
I have a worksheet with telephone expenditures and want to track them. but
some cells have a strange value, for instance one displays the following
value: 30:00:00 but in the formula field appears: 01-01-1900 6:00:00
Excel believes the value is 30 hours, but the right value is 30 minutes.
Other cells are OK, but when I add them, the wrong value alters the result
How can I fix the problem? I've tried changing the format or changing by
hand to the right value (30 minutes), but without success
Regards
Patricio Boric