Working with time sometimes it is really tricky, the solution is to FORMAT the cells with a Date-Time format
Second, Excel's default time format does not allow a time value to exceed 24 hours or 60 minutes or 60 seconds and that is why you end up with false results after doing some calculations. There is a workaround:
The common format for time is HH:MM:SS (You have a colon between the HH MM and SS format). To:
• Express a time exceeding 24 hours
Use a format placing brackets around the hour [HH]:MM:SS.
• Express a time exceeding 60 minutes
Use a format placing brackets around the minutes [MM]:SS without the HH format
• Express a time exceeding 60 seconds
Use a format placing brackets around the minutes [SS] without the HH:MM format
HOW TO FORMAT:
Assuming you have a time data on cell A3
• Select the cell A3 and right-click it
• Choose Format Cells.
• In the Format Cells dialog box, click the Number tab.
• From the Category list (left side list) choose Custom
• Type [HH]:MM:SS or other time format into the box labeled Type
(Tip: if you want no seconds then omit the SS format parm)
• Click OK.
After you have define your format you can do any calculation with the data
Remember that changing a cells format does not alter its true underlying value, it only alters the way excel "shows" the value