What does “coherent mental reckoning” mean in this sentence?

Question

Our units of temporal measurement, from seconds on up to months, are so complicated, asymmetrical and disjunctive so as to make coherent mental reckoning in time all but impossible.Indeed, had some tyrannical god contrived to enslave our minds to time, to make it all but impossible for us to escape subjection to sodden routines and unpleasant surprises, he could hardly have done better than handing down our present system. It is like a set of trapezoidal building blocks, with no vertical or horizontal surfaces, like a language in which the simplest thought demands ornate constructions, useless particles and lengthy circumlocutions. Unlike the more successful patterns of language and science, which enable us to face experience boldly or at least level-headedly, our system of temporal calculation silently and persistently encourages our terror of time. ...

From Robert Grudin, Time and the Art of Living.

What does coherent mental reckoning mean in this sentence?

Answer 1

Reckoning is another term for counting, reasoning, or (in this case) calculation.

Mental means "performed by, or existing in, the mind"; in this case, doing the reckoning "in your head", without the aid of external devices like a calendar or paper and pencil.

Coherent means "logically connected" or "consistent".

Putting them all together, "coherent mental reckoning" means "the ability to reliably and correctly perform date/time calculations in your head".

(And it's very hard to do that with the mish-mashed conglomeration of units, sizes, and exceptions that our calendar consists of.)

Answer 2

How I read the quoted statement: For various reasons, both physical and psychological, it is difficult for people to accurately count and judge the passage of time. Our odd mixture of units for measuring time (from very brief to very long) make this an even more difficult task. The original is expressed in rather flowery and complicated language, rather than being straightforward.