I've never tried to read the OE rules cover-to-cover. They're too brain-fragging. So I can't comment on those.
In Gygax's AD&D the stuff on time in the campaign seems to rest on some assumptions, in the sense that it only makes sense relative to those assumptions:
(1) A standard expedition to a dungeon is no more than 1 day;
(2) The play group consists of various participants who may or may not be present, and who pick from a stable of PCs (this is reinforced by the discussion of Successful Adventures in his PHB);
(3) The campaign is being GMed frequently enough that a rule of ! day in game for each real life day for those not on wilderness treks makes sense.
Then there is the discussion (spread over multiople sections of the DMG) of nerfing the magic items of PCs ported in from other GM's worlds.
None of this made any sense to me as a relatively new player who was running a single campaign in a non-club context. So I mostly ignored it. I can make sense of it in retrospect because of my improved knowledge of how Gygax and others played the game in those early days. But I think that approach to campaigns is probably now a minority approach and I wouldn't expect any modern DMG to say much about it.