HI PW77,
What you are missing is the distinction between "time and date," which in Numbers means 'time of day, and includes the date, of a specific instant in the passage of time, such as 9:41AM on August 25, 2022 (the time in California at which you posted this question), and Duration, which is a quantity, (measured in weeks, days, hours, minutes, seconds, and milliseconds) expressing the difference between the date and time which something started and the date and time that same something ended.
TIMEVALUE is a function that ignores the Date part of a Date and Time value and converts the Time part to a decimal fraction number representing the fraction of the day that has passed between midnight and the value in the Time part of that Date and Time value. Multiplied by 24, the result is a number representing the number of hours and fraction of an hour that has passed in that time, which you've used, with the rates in column C to determine the billing amounts in column D.
Your TIMEVALUE formula could be greatly simplified to: TIMEVALUE(B2)*24
While that works, it doesn't record the data you want to record.
Making 'time of day' stand in for 'duration' will always give you the undesired Date part of a Date and Time value in your entries in column B.
Formatting those cells to contain duration values, or simply entering the values in a format Numbers will reliably recognize as a duration, will make things simpler and less error prone.
Enter the durations in B2-B4 as:
1h 7m 11s
1h 59m 20s
1h 32m 22s
Enter Rates as they are now - numbers formatted as currency.
In D2: DUR2HOURS(B2)*C2
Fill down to other rows containing 'Task' items.
Moving the Subtotal calculation to an added column will simplify the Grand total sum, which will then be the simple total of the column containing the individual task amounts or of the column containing the subtotals.
OR you could leave task amounts and subtotals in the same column, use SUM(D)/2 (in a Footer Row) to total that column, and divide the total by 2 to counteract the fact that the grand total adds each task amount twice—once as a task amount, the second time as part of the subtotal in which it is included.
Regards,
Barry