I'm actually fairly certain it was "within working hours" free time. Facebook doesn't have 20% time or anything like that, but, in practice, there's lots of extra time for whatever you want. This was my experience as an Engineer at Facebook and I certainly "pay it forward" as a manager.

Probably worth adding that we ultimately created a python foundation team, of which Jason is a member.

Here's something dumb I created in my "within working hours" free time that Facebook let me open source: https://github.com/facebookresearch/py2bpf