A refinement to fix the Time::strptime lack of %U/%u/%W/%w/%V/%g/%G support pre-2.7 — see https://github.com/ruby/ruby/commit/a9d4f2d03c847ec1c89dc03a5076a9fa29ffa61f#diff-0ead6351265632d68f106a2b1a98f5f9

fix_strptime.rb

require 'time'

module FixStrptime
  refine Time.singleton_class do
    def strptime(date, format, now=self.now)
      d = Date._strptime(date, format)
      raise ArgumentError, "invalid date or strptime format - `#{date}' `#{format}'" unless d
      if seconds = d[:seconds]
        if sec_fraction = d[:sec_fraction]
          usec = sec_fraction * 1000000
          usec *= -1 if seconds < 0
        else
          usec = 0
        end
        t = Time.at(seconds, usec)
        if zone = d[:zone]
          force_zone!(t, zone)
        end
      else
        year = d[:year]
        year = yield(year) if year && block_given?
        yday = d[:yday]
        if (d[:cwyear] && !year) || ((d[:cwday] || d[:cweek]) && !(d[:mon] && d[:mday]))
          # make_time doesn't deal with cwyear/cwday/cweek
          return Date.strptime(date, format).to_time
        end
        if (d[:wnum0] || d[:wnum1]) && !yday && !(d[:mon] && d[:mday])
          yday = Date.strptime(date, format).yday
        end
        t = make_time(date, year, yday, d[:mon], d[:mday], d[:hour], d[:min], d[:sec], d[:sec_fraction], d[:zone], now)
      end
      t
    end
  end
end

using FixStrptime

p Time.strptime('202001', '%Y%U')
#>> 2020-01-05 00:00:00 -0800