How to use calculated columns to assign a calendar Datetime span from a choice field

Say you have a production planning with guys working with 4 different schedules :
A. 7:30 to 16
B  9:30 to 17h30
... etc

With standard sharepoint, you have to create two dates (begining and end), select they day twice, and type the hours each time.

You want to only write a date and select the time span :

Resolution :
Create a choice (C_schedule), prefix each choice with "1", "2", "3", etc....
Create a date field (D_work_date, without time),
and two calculated columns of datetime result: DT_work_start, DT_work_end

You'll make sharepoint fill those columns with basically, the start date + the hour(s) of your working day.

The formulas will be

#START TIME
DT_work_start=D_work_date+IF(LEFT(C_Schedule)="1";"07:30:00";IF(LEFT(C_Schedule)="2";"07:30:00"; ..... )))

#END TIME
DT_work_end=D_work_date+IF(LEFT(C_Schedule)="1";"16:00:00";IF(LEFT(C_Schedule)="2";"17:30:00"; ..... )))


Et voila! :)