Selected Answer
Excel cells have many poperties, among them the Formula, the Value and the NumberFormat. For example, if you enter =TODAY() in a cell Excel will know that it's a formula by the leading = sign and assign the string to the Formula property automatically. Then Excel will evaluate the formula and assign the result to the same cell's Value property. In this case, today, that is 43503. While Excel evaluates the TODAY() function it learns that the result must be date. Therefore a NumberFormat like mm/dd/yyyy will be set (depending upon your computer's Internationl settings), also automatically, and you may see 02/07/2019 displayed in the cell.
You can reverse the automation in each step. You might set a NumberFormat like dddd and display Thursday in place of the date. Or set the NumberFormat to General and see 43503. You might precede the formula's = sign with an apostroph, like '=TODAY(), and thereby cause the formula to become a text string which Excel will assign to the Value property and no longer evaluate.
Today is the 43503rd day since Jan 1, 1900. It follows that tomorrow will be represented by number 43504 and yesterday was 43502. =TODAY()+10 returns 43513 which can be displayed as whatever the NumberFormat property demands, and that is the entire "secret" of calculating dates in Excel.
Since one day is represented by the integer 1, it would follow that 0.5 should be half a day. In order to instill this notion with sense MS engineers determined that day 43503 should start at 00:00 and end at 24:00. Therefore any fraction added to 43503 must represent a time during that day. Logically, 43503.5 should be 12 noon. And it is.
Accordingly, if you apply a NumberFormat like hh:mm:ss to the cell containing the =TODAY() formula you will see 00:00:00 but =TODAY() + 0.5 would display 00:12:00. Try =TODAY() + 0.75 would display 00:18:00 or 00:06:00PM. If you have =TODAY() in A1 and B1 =A1 the two cells could have the same Value but display different things. You can also construct Date/Time NumberFormat like mm/dd/yy hh:mm:ss.
Since the interger 1 = 1 day, and one day = 24 hours, 1 hour must be 1 / 24. 1/60th of that (1 / 24 / 60) = one minute, and so on. Excel applies the conversions as part of the NumberFormat. You don't need to know or care how many threes there are in eight hours. The challenge you face is focused on data input. I have suggested that you use date/time numbers for all your times throughout your project. Therefore every time you enter must be converted to its appropriate date/time value at the point of entry.
Actually, Excel makes this task easy, again employing the NumberFormat. A cell formatted with hh:mm:ss will be assigned the correct Value automatically if you enter 8:00 or 8:00:00. Use Data Validation to reject entries like 8 or 8.00. Find a way (using hidden columns, perhaps, or VBA) to add the date to the user's entry.