@kyujyu - think you mistyped that or misunderstood something (or I am not seeing a different way to read it with different punctuation or something... My bad if this is the case, but not seeing it even after multiple rereads.)
"1 day inside = 20 days outside, not 1 day inside is 20 days outside" - You first say 1 day inside is 20 outside, then you say it's not 1 day inside is 20 days outside...
Anyways, to others asking (
@animecruizer,
@kuri_something), to clarify, yeah, it's 1 day of 'outside ('normal world') time = 20 days spent inside ('training dimension or whatever') time. Should be anyway... I can't read moon well so I can't check raws to be sure. If inside had time 'slowed down' (as it mentioned) it would be less time inside = more outside, which wouldn't exactly be useful when trying to fit more training time into less 'real world' days.
He gets 20x the training time, while only a single day passes for those in the 'real world.' This would allow him to fit in much more training on those 'star' days when it's usable.
At least, that should be how it works based on everything said so far. The translation could have mistakenly switched slowed/sped up time, or something like that, and/or then accidentally swapping inside/outside at some point. If inside had time slowed down compared to outside it would be wasting his training time; you'd go inside and train for '1 day,' from your perspective, but wasting 20 days worth of his limited training time available.
I'd say it could maybe work that way, if another special trait of that space did something else, like greatly increased his trainings results, but thats not a subject even brought up. And it'd need to increase a LOT to make using up 20 days worth it for just a single 'useable' training day.
Hope this helps, sorry if it's not the easiest explanation to understand, been a long ass day.