irb(main):056:0> Date.strptime("5/21/19", '%m/%d/%Y')
=> #<Date: 0019-05-21 ((1728138j,0s,0n),+0s,2299161j)>
irb(main):057:0> Date.strptime("5/21/19", '%-m/%d/%Y')
Traceback (most recent call last):
        3: from /usr/bin/irb:11:in `<main>'
        2: from (irb):57
        1: from (irb):57:in `strptime'
ArgumentError (invalid date)
irb(main):058:0>