whatever it is, that is how i remember the 90s.
I grew up in the 90s and 90s is the best decade of all.

It's the same with the Burton films or the new series Gotham.

"If you are unfamiliar with the series (though I'm not sure how anyone could possibly find out about this movie without knowing about the series)"
I'm aware of it, but I haven't seen much of it.
"it was developed with a "dark deco" aesthetic, while still depicting modern technology, for an overall feel that is timeless and hard to place in the real world."
They did that very well.

This question is easy Mask of the Phantasm takes place during Batman: The Animated Series.
MotP was the movie based on the animated series, which arguably best captured the spirit of the Batman universe out of all the various onscreen depictions.
If you are unfamiliar with the series (though I'm not sure how anyone could possibly find out about this movie without knowing about the series), it was developed with a "dark deco" aesthetic, while still depicting modern technology, for an overall feel that is timeless and hard to place in the real world.

Like the creators have said in the commentary features for "Batman: The Animated Series", time periods and technologies are mixed and matched in an attempt to give the series and spin off films a timeless quality (I think they succeed; Computers with a film noir and art/dark deco look really works for me).

I had assumed that it took place before Dick Grayson was adopted and before the Joker met Harley. But novelization tells me otherwise that Dick is now a college student.

You mean that the 40's NEVER went out of style?

This message has been deleted.

That's one of the things I love about the Animated Series. It's its own little universe, with stuff from so many time periods a part of modern everyday life.

Oh, I wasn't aware of that. It would also explain the computer Batman uses and the fact that a black woman hits on him and no one gives them dirty looks.

This message has been deleted.

Good theory.

One could say it takes place in an alternative 1990's where art deco never went out of style.