When you say you are trying to type in the number 8-11, do you mean that you are typing '8-11'.
If you are entering =8-11, Excel will assume that you are entering 8 minus 11 which is -3
If you enter '8-11', Excel will assume that you are entering a date and return 8 November.
However what shows in the cell will also depend on how you have formatted the cell.
If the format is hh:mm you will get ###### for -3 as Excel will not accept negative times, or 00:00 for 8 Nov as there was no time entered (as far as Excel was concerned)
If you want to enter a time in a previously unformatted cell, enter it as 8:11 which will be recognized as a time and if formatted as hh:mm will show as 08:11.
If formatted as h:mm AM/PM it would show as 8:11 AM
Can you explain why you expected 8-11 to show as 8:00 AM and not 8:11 AM or 12-6 as 12:00AM
You also say I have tried many formulas to do this.
can you post what you have tried.
Usually just entering a time as 12:06 or 11:45 with no formula, no equals sign and just a colon between the numbers works OK. Then adjust the format using the normal formatting, such as h:mm AM/PM or hh:mm
As an aside, Excel stores times as a decimal value. 0.5 is 12 noon, i.e., half way through the day. Dates are stored as the whole number part of the value, so 40336 is 07, July 2010 (1 was 01 January 1900 in Excel running in Windows OS's)
40336.645833 is 15:30 on 07, July 2010