So, when it says approx. next to the completion date does that mean it really could be early or late? Is like a day variance? It would be better to have a day earlier for my game is why I am asking. Made the mistake of building too much the first day I got my team.

12. What are the construction times for the the facilities that we can build?
Stadium upgrades and Youth academy take 15 days to build, the club shop is constructed in 7 days, training hall in 14 days and the fitness centre in 10 days.
No variation from everything i know.

the reason it says approx is because the Stadium will upadtes at 00:30 am every single day, at that time regardless of whether you built it early in the day or late in the day

that means if you make a a stadium order at 23:00 PM 4-1 (month, day) which is just before the day ends, you will actually get your stadium 14 days and 1h 30min after the order (4-15 at 00:30 am)
if you make the order at 1:00 AM 4-1 which is just when the day started you will have to wait 14 days and 23h30 min (4-15 00:30 am)
so its exactly the same thing. in the second case you made your order early in the day and spent that 22 hours (that separate the order times) already with an order in. the first case is when you still technically spend that 22 hours but just not yet ordered.
the wait time is the same for everyone if you think about it. one just orders and then wait, the other waited before then ordered

at least thats what i think it means