The phrase "a milestone when" is correct and usable in written English.
It can be used to indicate a significant event or achievement that occurs at a specific time or under certain conditions.
Example: "The project reached a milestone when we completed the first phase ahead of schedule."
Alternatives: "a significant point when" or "a key moment when".
North-South relations appeared to reach a milestone when a pact of reconciliation and nonaggression was signed in December 1991.
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.
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.
This year, one of the biggest dam removal projects in the nation's history reached a milestone when a section of the Elwha River near Olympic National Park in Washington was restored to wild flow, with fishing rights an important driver in the process.
The owners, who are already derided here for spending money on luxury homes rather than on new players or security, were in danger of not meeting their payroll; indeed, it was considered something of a milestone when Mr. McCourt was able to meet his payroll at the end of May.
There was a even a nod to a long-stalled Nasa mission to Mars, which Obama said would soon see a major milestone when a US astronaut spends a year on the international space station to test the effects of long-term human habitation in space.
In January, Russia reached a sad milestone when a 26-year-old woman from the Rostov region became the millionth person officially diagnosed with HIV.
The investigation of the Columbia accident reached an important milestone when an independent board headed by Harold Gehman, a retired admiral, issued a "working scenario" of how the accident might have happened.
The pork episode marks the book's first use of the concept of dreaming in Chinese, which comes from the popular notion that you've reached a linguistic milestone when you start dreaming in a foreign language.
In September, the college passed a significant milestone when the Israeli cabinet voted to upgrade the college to a university as a matter of "national importance".
