A very common obstacle to professional development is a lack of time. While this is a genuine issue, the reality is we can't change the fact that there are 24 hours in a day, 7 days in a week and, and 365 days in a year (although we get a bonus day next year!)
What we can change, however, is how we use the time that we do have. What if we organized schools differently so that cohorts of students were grouped with a cohort of teachers? When a teacher from the cohort needed/wanted to participate in a PD opportunity, they wouldn't have to worry whether a competent sub (or any sub for that matter) was available. They could participate in the PD opportunity, while the other teachers from the cohort would figure out how to best use the available time with the students.
At the high school level, what if students in each cohort spent one day a week out in the community in a work-based learning opportunity? This would give the teachers in that cohort one full day a week to collaborate and participate in professional learning opportunities.
These are just a couple of possible examples. The primary point is that we need to think differently about how we structure schools to best utilize the scarce amount of time that we have.