I agree with Faldage here: it's not wrong to use is when when you're actually talking about a time! Maybe if you can flip it around into a sensible question-and-answer pair it's OK:

When is our truest life? It's when we are in dreams awake.

as opposed to

When is an orbit? It's when one object in the universe (for example a planet or a star) goes
round another one without touching it.

The second one should be what is an orbit!!!! Hence you cannot use when in the original version either. Well, you can, but it is informal and - well, maybe infantile* is a bit harsh - to use it to precede a definition which has nothing to do with time.

* But little kids speak like this all the time, so infantile is probably correct.