Gets a double number of man-months that represents
duration. The number of man-months is computed from
this.
DebtSettings.
DebtSettings.NumberOfWorkHourPerDay and
this.
DebtSettings.
DebtSettings.NumberOfWorkDayPerYear.
Target Platforms: Windows 11, 10, 8, 7 and Linux, MacOS with net8.0, net7.0 or net6.0 installed