The reason why UPS doesn't say when the end of day for delivery is for them to deliver packages is because the UPS will usually stay out even late at night until all packages are delivered to the addresses.
That means that the UPS driver will not end his day until he delivers every package on the UPS truck to you so there's no set time that the UPS ends the day.
So that's why that UPS can't state a time for end of day.