When do the extra turns from Time Stop happen?

Time Stop says “you take 1d4 + 1 turns in a row”. My question is, when do the additional turns happen? Here are two scenarios I can think of.

Scenario 1: The caster casts time stop. This immediately ends the caster’s current turn and begins the first of the additional 1d4+1 turns. If this was the first action at the beginning of the caster’s turn, he effectively loses any potential movement, bonus actions, or even additional actions (if he were hasted) on the turn he cast time stop.

Scenario 2: The caster casts time stop, which immediately freezes all other creatures, but the caster can still complete his current turn and all associated actions. The first of the additional 1d4+1 turns begin after the current turn is complete.