Can you cast a sorcery-speed spell during your opponent's turn if you have Teferi, Time Raveler on the battlefield?

Teferi, Time Raveler's static ability has two distinct effects: (1) each opponent loses the ability to cast spells at instant speed, and (2) you may cast spells with sorcery timing as though they had flash. Neither of these effects grants you permission to cast spells during your opponent's turn.

Under CR 307.1, sorcery spells can only be cast during your own main phase when the stack is empty and you have priority. Teferi's ability to let you cast spells 'at any time you could cast a sorcery' does not change when sorcery timing is legal — it only allows you to cast instant-speed-eligible windows as sorceries. It does not turn sorcery timing into instant timing for you.

What Teferi does do to your opponents is prevent them from casting spells or activating abilities at instant speed (CR 116.1, 509.1), effectively limiting them to sorcery speed as well. But this restriction applies to them, not as a benefit extending your own casting windows.

Concrete example: You control Teferi, Time Raveler. During your opponent's turn, you cannot cast Divination (a sorcery) simply because you have Teferi. You must wait until your own main phase with an empty stack to cast it.

To cast spells during an opponent's turn at sorcery speed, you would need an effect that explicitly grants you that permission, such as Vedalken Orrery, which grants flash (instant-speed casting) to all your spells — a fundamentally different effect (CR 702.2).