Time is passing anyway or is it just an illusion?

If you choose to measure it then it matters about
the method and technicque to use.
The ancient one is the day, the epoch, the year.
Since the day is defined between two sunsets and
these sunsets come late one second every 1 or 2
years then the correction is significant.

What happens if a missile is launced 1 second later
or earlier, will it find its target ?

A drift like this one third of a second is catastrophic.
Are you aware of famous time drift accidents like this:
http://en.wikipedia.org/wiki/MIM-...at_Dhahran

"the failed intercept at Dhahran had been caused by a software error in the system's clock.
The Patriot missile battery at Dhahran had been in operation for 100 hours, by which time
the system's internal clock had drifted by one third of a second. Due to the closure speed
of the interceptor and the target, this resulted in a miss distance of 600 meters.

At every place where corrections can be made, then these must be made.

K.L.Angelis