The phrase "a milestone when it" is not complete and lacks context, making it difficult to assess its correctness in written English.
It could be used in a context where you are discussing a significant event or achievement that occurs at a specific time or under certain conditions.
Example: "The project reached a milestone when it successfully passed the final review."
Alternatives: "a significant point when it" or "a key moment when it".
Earlier this year, Tesla hit a milestone when it delivered its 200,000th electric vehicle.
Last year his team achieved a milestone when it manoeuvred a 1.5-millimetre bead through a 5-millimetre artery in a living pig.
This month, Credits passed a milestone when it became the exclusive payment method for most of the games created by Zynga, the No. 1 developer of Facebook applications.
I very much welcome the judgement of the Court, which will hopefully be a milestone when it comes to online privacy.
On Tuesday, the Department of Transportation marked a milestone when it issued far-reaching — yet flexible — guidelines that pave the way for self-driving cars to hit the roads without much red tape.
Mercedes-Benz also recently reached a milestone when its autonomous bus completed a 12-mile journey.
Earlier this year, Tesla hit a bittersweet milestone when it delivered its 200,000th electric vehicle.
The W.T.O. also passed a significant milestone when it admitted China and Taiwan as its 143rd and 144th members, meaning that its membership now covers all the major industrial and developing countries with the exception of Russia.
Spotify recently reached a major milestone when it reported that 52percentt of its music consumption happens on mobile.
Netflix passed a pretty significant milestone when it comes to its international expansion plans: (slightly) more than half of its subscribers now come from outside the U.S. Its users are watching around 1 billion hours of content per week (that's billion-with-a-B).
But it was still a milestone when, this month, the conglomerate opened its sixth and most expensive resort in China's preëminent financial hub, Shanghai.
