I noticed that this pattern is often used by English speaking Russians. How does it sound to a native English speaker? Usage example: "I could do that, but not earlier than in a month".
"I could do that, but not earlier than in a month."
Most of the time, I think I would change the perspective, and get rid of the negative:
"I could do that, but it would take at least a month."
Otherwise, I might use sooner rather than earlier:
"I could do that, but no sooner than a month from now."
It is unnatural, if not ungrammatical. "Earlier" is comparing to another time, but "in a month" is not a time, it's a property that a time could have. You can say "faster than in a month", or "earlier than a month from now".