It is indeed difficult to classify death into Kala-mrityu (timely death) and Akala-mrityu (untimely death), other than perhaps for awakened folks.
The nature of the physical body is death and decay - so most have to die. Death as such here would be “timely death”. Now, what is “timely” for a given individual? Hard to say. Some folks attribute it to karma, some others to a pre-determined span of life (ayus), etc. But do I know for sure, not really haha… I do however know that there are many alchemists and yogis who can bend this law of “timely death” and extend lifespan. My Daoist teacher is “very” old and hides in Kunlun mountains (he has great great grandchildren lol). Another one has “changed” bodies (I’ve seen it once) and extended life span. So there’s that stuff too lol
As for untimely death, it is death before one’s soul purpose is accomplished, so I would think in such cases the choice is not really the soul’s. Suicide is one such thing (although there are complexities here based on different cases). This can be avoided by Divine Grace (I wrote about my experience here).
So, what is timely vs untimely, what causes either of them, is one’s lifespan pre-determined? I can only speculate as I have no clue 
Mrityunjaya can help with both - mitigate the untimely death, and in the case of timely, mitigate the fear of death (aka limited existence where the self identifies itself as limited to the Body-Mind complex).
The mantra literally says - lead me from Death to Immortality (can be physical immortality, or of the dawning of immortal consciousness that already IS, but not recognized due to all the dirt that covers the mirror of consciousness).