USPS expected delivery time is usually and often pretty accurate.
However sometimes the USPS may have delays which prevent them from delivering your package on time such as bad weather, icy roads, being short staffed and not getting to your package on time to get it out on the truck to your post office.
I've had some packages that arrived a day or 2 later than what the expected delivery time was.
However most often I get my packages delivered on the day it is expected but the date can sometimes change.
USPS distribution centers work 24 hours a day and 7 days a week.
The mail is always being sorted and distributed 24 hours a day and 7 days a week even though your local post office is closed.
Truck drivers haul the mail in between USPS distribution centers where workers at the USPS distribution centers then unload the trucks and sort the mail.
Other workers load sorted mail onto other trucks to travel again to another distribution center or post office.
If the USPS didn't sort and haul mail 24 hours a day and 7 days a week they would be overworked and over run by mail.
Most mail leaves the USPS distribution center within a few hours to a day although sometimes the USPS may have way too much mail that it may take 2 days to get to your mail.
This is more true as it nears the holidays such as Christmas or other holidays.
The more mail they have to sort the longer it will take and sometimes some employees may be out sick and so they get short staffed.