There are lots of ways to handle this, but the essence is the same – if your work times are going to cross over from one day into the next, then the day must be part of the format, or it will be difficult to do calculations.
Excel’s floating-point datevalue.timevalue format, where the integer portion represents the date, and the fractional portion represents the time (as a fraction of the day) is one way to do it.
Another is the military-style YYYYMMDDhhmmss format, which has the advantage of being easy to handle either as a number or a string, and sorts easily.
A third variation is the modified-Julian date format with time, YYYYJJJhhmmss. Julian day is simply the day of the year numbered consecutively, so 001 is January 1, 032 is February 1 (the day after January 31), 060 is March 1 in a regular (non-leap) year (31 days in January, plus 28 in February, is 59, so the next day, 60, is March 1) and so on. 001-365, or 366 in a leap year. Or if you want, you can go to standard Julian dates, used by NASA and other agencies who track things like orbital data.
Julian date can be very handy for planning and calculation, because it’s very easy to add and subtract: What date is 90 days from 2010107? Answer: 2010197. Now, without looking I couldn’t tell you what the calendar day was for either of those, but when you’re attempting to calculate durations, Julian can be handy.
And once you change to a date-time format, Min() and Max() become very easy, regardless of which system you use.
Also remember that what your system uses “behind the scenes” does not need to be what the user sees routinely: in fact, it probably shouldn’t be. Type a date into Excel, then override the cell format to a floating-point number. You’ll have no idea what the number means. (Neither would I) – but so what? Excel handles it internally, knows what it is, and how to deal with it. Your system can be the same.
So the clock says “Oct 12 10:00” but internally it’s “20101012100000” or “2010284100000”
Format the date to the user for display and printing, but deal with it internally as a complete date-time datum.