Not trying to be funny or offensive in any way but have you considered getting a book on Excel. The library has a few copies so you don't have to buy one. I did notice one of my local bookshops, can't remember the name of it but its one of the cheap stores not Waterstons etc, is selling PC books very cheap and one of them was for Excel. Did actually consider buying it as its one of the excellent Dummies books which normally retails at about £15. Amazon are selling it for a tenner click here but the bookshop I am refering to are only asking about £4. Well worth it if you use Excel a bit and they have a few other books as well.
Sorry I can't remember the stores name, if I do recall it I'll post back.
How are you getting 23:50? Are you adding them instead of subtracting them?
More specifically, do you just want the difference between the times, regardless of which is the earlier, (the first given or last given?). We might need to accomodate a "negative" difference, which Excel can't display...
And are they times as opposed to durations, (e.g. 12 O'clock rather than 12 hours)? It can make a difference to the needed formatting. To make it slightly clearer... there is no such time as 25 o'clock, but there is a duration of 25 hours.
Post back withthe answers to these if you need more assistance.