Yes, there is a format called Duration. But, it does not add a date. If you are getting a date added, you are not actually in the Duration format, but rather the Time and Date format. When only a time is being displayed it looks just like the colon-delimited duration format. If you use the Cells Inspector, you will see which format is in effect.
Delete the cell content, change the format, and reenter the data. That should put you on the right track.
I am now back after days; I brought my one task to an end by adding the values (just ~~50) in my head; it was a special kind of training, not planned
I think I can work with the duration format now, and I found to subtract date (it is the running, today) and to froze the result by cut and paste values.
It is somehow quaint to cannot give cells a format at my needings. Though, I know on the other hand, each mystery has its reason; and the feedback to apple is easy to find…
Have nyce days,
the Numbers'09 User Guide explains Date and Time formats and Duration formats (pages 95-96). The Guide is available for download from the Help Menu in Pages. It is searchable. Also the Formulas and Functions Help from the Numbers Help Menu. Also the Function Browser (click on the Function button on the Toolbar in Numbers).
As Jerry said, the Inspector > Cells Tab will show the cell format (start reading on page 20 of the Numbers User Guide).
I know that it can be confusing. Some cell formats will be changed, at least temporarily, if the result being displayed comes from an operation that clearly demands a different format. However, in some cases it may not be clear which of multiple possible formats the user would prefer. Formatting in advance generally solves this problem. I'm glad you found a solution, even if not an optimal one.
there is no trouble if I have a little calculating sheet into which I fill some content with keystrokes.
Last time I imported nearly 50 records as the descriptions of the songs of an opera, wanting too calculating the length of the acts as the sum of the song-lengths.
50 records are not so much, thousand are usual coming from databases, think for N S A ;)
Clearly numbers has nice features too. (At my youth I was mastering Tabellenkalkulation and was one of the strongest users of ms excel in my country, really ;))
~ br, Karl