Since years I am trying out whether one of the spreadsheets (excel, Lotus 123 
or now OO) is able to program the cell formatting as a clean user interface 
(as it should be by definition) A good example for what I consider to be 
clearly wrong in that regard are most time definitions - whether chosen from 
the list or user-defined. 
E.g. if I want a cell to contain minutes and seconds only there is no way to 
succeed!! That means that there is no way to achieve that the contents can be 
entered and is visible as minutes and seconds only. If I choose 37:46.00 
(accepting decimals to the seconds) or if I enter a user-definition of mm:ss I 
still have to enter the hours with "0:" and if I enter the minutes or 
calculate into this cells minutes beyond 59 the spreadsheet program splits the 
hours away and drops them - a clear contradiction to the cell definition: I 
have said that I want a cell with minutes and seconds and never have mentioned 
the (hidden) hours!
When I enter a definition of mmm:ss, which should make the matter finally 
clear (although a value beyond 60 would not yet require a third position) 
another problem arises, which is a clear bug. According to the help text m-
minuscules are minutes, M-majuscules are months. But all my minutes 
definitions are shown in the Format Code fields as M-majuscules thus meaning 
months and my mmm definition is also interpreted as such. Somewhere in an 
application their might be differences between minor and capital letters and 
your software should consider this where it matters.

With best regards
Bruno Naumann

P.S. If you separate the functionality of the cells (internal value for 
calculating, sequencing etc) from the user interface (=cell formatting) as it 
should be, you would be miles ahead of MS which throughout their entire 
software in numerous instances makes an incredible mess between the two.