public IFluentOperatingTime TotalHours()
 {
     calculationUnits = CalculationUnits.Hours;
     return this;
 }
 public IFluentOperatingTime TotalMinutes()
 {
     calculationUnits = CalculationUnits.Minutes;
     return this;
 }
 public IFluentOperatingTime TotalDays()
 {
     calculationUnits = CalculationUnits.Days;
     return this;
 }