In MDL-48762, dobedobedoh raised a few ideas to improve how Moodle is dealing currently with start/end dates.

He proposed a different approach to improve the current enrolments/course API and add some settings to the course level to define the default enrolment start/end dates too to be inherited by the enrolment plugins (that could be overridden later by teachers if required).

His proposal is to create new settings in the course settings for:

• Enrol [role] [period] [before] course [start|end] date
• Unenrol [role] [period] [before] course [start|end] date

For example, if we had three-course roles, you'd have:

• Enrolments start (Heading)
• Enrolments for students start relative to course dates [checkbox]
• Enrolments for students start [5] [days] before course [start] date
• Enrolments for teachers start relative to course dates [checkbox]
• Enrolments for teachers start [5] [days] before course [start] date
• Enrolments end (Heading)
• Enrolments for teachers end relative to course dates [checkbox]
• Enrolments for teachers end [5] [days] after course [end] date

That's something that needs to be analysed by the UX team but I've created the issue to record for the future.