Problems I had with other similar packages

from and to inputs had to be discrete dates (can't specify exact times)
Could not have custom schedules (specify exact datetimes rather than "Monday from 14:00 to 15:00")
Weak timezone support: could not set timezones on a per availability basis
Outputted slots did not have to property, just a time
No TypeScript support (or even a declaration file)

Things to remember