This all happened during a time when employees were _not_ allowed to sell stock.

Twitter employees can't currently sell stock? I was unaware of this.

They can only trade one month out of three. Specifically, they can trade immediately after quarterly earnings.

This is pretty common amongst similar companies.

Is there a rolling window?

Yes, there was a stock sales window when I worked there and I assume there still is now.

Most publicly traded companies have lockout periods if you have access to "insider information." Given that Twitter is (was) fairly transparent about company financials, this is probably still true.