Arduino, Receiving Instant Time When Event Occurs
It's not very clear what you expect to see.

You are currently using millis() which returns the number of milliseconds since the program started running.  This can run for 50 days without wrapping.  It has no relevance to the real time unless you recorded the time the program started.  You could write something to add millis to the start time if you wanted (look at http://playground.arduino.cc/Code/time)

Alternatively you could use a real time clock (DS1307 or DS3221) which a usually just a few pence/cents.  These ICs keep the current real time and return it when queried.  If you are trying to log data from sensor (or cars) then this will give you the real time the data was received by the Arduino, and it will make a lot more sense.

Your current code will display

45678

An RTC could let you display

Wednesday 22nd February 2017 13:05:42.32