As @zhantongz points out, the 'future in the past' certainly allows the use of the 'not until' sense of only with 'events set in the (relative) future', and this includes dates as the change-point:
The savage, hideous war continued, and every right-thinking person, if
not actively engaged, looked on in horror. Appalled. But only on the
6th of August 1945 would the world become aware that mankind now had
the potential to destroy itself in perhaps a matter of days.
There is at least a connotation of there being something unfortunate / surprising / showing culpability / showing an unfortunate imperfection/falling short / mitigating, with 'only' in the temporal sense. (/ = and/or)
A final report reached him only on January 15 ...
- so he was unable to take the necessary steps to avoid the collapse of the company.
- and this in the age of computers.
- (those responsible for the late delivery have been reprimanded).
- so what happened next is hardly his fault.
And these connotations don't apply to a lot of statements about the future
We don't leave until September 17.
Examples like Greybeard's
You will only be able to play 'Queen Quong' after you have installed
at the very least strongly connote, and quite possibly conflate with, the 'not unless' sense (and are totally idiomatic).
I'd say that if the licensing conditions mentioned apply (and usually only then), temporal 'only' is a legitimate (and possibly the best) option. Excepting those with the 'only if' overlap, statements about the unknown future licensing 'only', especially with regard to specific dates / times, are rarer. But
I forgot to post the Christmas card to the Sackville-Bagginses in the
Shire; it will only get there on the 23rd of January 23rd – and that's
if there are no orc-raiders on the Greenway. [showing an unfortunate