I think the best explanation is that the same general rules regarding the definite article ("THE") and the indefinite article ("A") are at play, but it's harder to conceptualize with such an abstract noun as "time".
The best way to show what I mean is by example.
1-At the time when I was shepherd, one could only see wolves with binoculars.
The time when this guy was a shepard is a specific, discrete block of time. During that time, there was only one way to see wolves. (DEFINITE ARTICLE)
2-This is the time when I'm finally able to relax.
This one's trickier. Let's imagine what scenario when this sentence would sound right. You're sitting by the pool with a nice drink, as you do every day at 3 PM. This sentence is a somewhat exagerrated statement that this is THE specific time when you can actually feel relaxed. It may not be literally true, but think of how a weird a similar sentence would sound with the indefinite article: You're at a great bar, and you want to tell your buddy next to you how cool you think it is. "This is THE place to be!" is more idiomatic than "this is A place to be", which while technically correct, is weak and meaningless.
3*-There used to be a time when I loved my mom. (Isn't she or he talking about a specific time?)*
It's not specific enough to use the definite article. If the sentence were "Last month was the time when I love dmy mom", then you'd be right. But the speaker is just saying that a time when that was true used to exist. (INDEFINITE)
4-Anna Akhmatova lived at a time when poetry mattered.
There may be many different times in history when poetry mattered. We don't know which one Anna lived during, but she lived in one of them. (INDEFINITE)
N.B. - There is also an idiomatic usage worth noting. For example, if a judge asked you whether you were employed when you robbed the liquor store last month, the idiomatic English response would be "At the time, no".