I think some of the dates come from BC, while the others come from the ultimania. Then, there's stuff you can figure out by using math on the available reference points to narrow down the time frame.
For example the last time I had a look at the game, I estimated a minimum of 38 days for the entirety of the game, with potentially another 20 days worth of travel time, depending on how long it takes to get from one place to another using various means. After returning to Midgar, they say there's a week until Meteor hits and then you get the Highwind scene, which is the last night. That part seems to imply everyone got a week to go wherever rather than just a day like what Cloud says seems to suggest. Either that or the plot skipped a week between those two moments like it did between Sephiroth summoning Meteor and Tifa waking up in Junon.
Considering how Zack was trying to get to Midgar, I figured Cloud just walked in, ended up at the train station and then collapsed where Tifa found him.