What the best option for designing the follow, taking into consideration that a planned job must have a startDate, startTime, endDate and endTime. The start can be entered days or weeks before the end and its important that the user does not forget to enter any of the fields at the appropriate time. There are also several calculations done on these fields sometimes where the date and time needs to be combined and sometimes when they need to be separate.
It seems like C most accurately represents the business rules you described. The C model prevents Dates being entered without Times (or vice versa). It also ensures that StartDate must be entered before EndDate.
I'm assuming "ID" is an identifying foreign key in the JobStartInfo and JobEndInfo tables, ie. ID is both a candidate key and a foreign key so that the relationships between the tables are 1 to 0/1.