There's a lot of talk about ear training for EQ and almost none about training for time. Yet it's half the job: a production with perfect frequencies and stiff groove sounds like a demo, while a mediocre mix with good feel holds up on its own.
The problem is that rhythm is almost never practised with measurement. You play along to the song and assume it's fine. Here the accuracy window is objective: you know in milliseconds whether you were early or late, and that immediate feedback is what sharpens perception in weeks instead of years.
One note that carries over to mixing: after hours with a song you stop hearing its pulse, because you know it by heart and your head fills it in even when the audio isn't. It's the same phenomenon that stops you noticing you have too much low end. A short rhythmic exercise on unfamiliar material resets that reference, which is why it pays off more as a warm-up before a session than as a diversion afterwards.
Rhythmic hearing is one skill on the full map, which lives in the ear training guide. To work tempo and key on your own files, the BPM and key detector.