Oh, it is all clearly (/sarc) explained in the AD&D Manual of the Planes:
"Time is constant in the known planes of existence ... the length of a day in one plane is the same .. in another plane. This type of time is called "true time". While time and the perception of its passage do not vary ..., the effects of time on living objects do vary widely. The time that regulates the differing planar effects on living beings is called "subjective time". Subjective time governs long-term processes that affect the traveler's metabolism and natural functions. This includes poisons, potions, natural healing, the effects of paralysis or aging, the need for sleep and food.
TRUE TIME vs SUBJECTIVE TIME:
Prime Material: two times are the same
Ethereal: 10 true rounds pass for one subjective round
Astral: 365,000 true rounds pass for one subjective round
Outer Planes: two times are equal"
Do you understand any of that? I do not. We'd probably be better off with a Lorentz Transform basis.
