Student or Learner
Please, explain the underlined expression :
Last week, just three weeks shy of my son's enlistment date, he underwent a high-profile security check under the auspices of the navy.
It is normally used when something is a less than needed or less than expected. It can refer to physical distance, distance in time (like your example), or distance in some other sense.
Some examples from news headlines:
1674-pound pumpkin just shy of world record.
Democrats lose ground, now eleven votes shy of passing Obamacare.
Many early boomers could run shy of cash in retirement.