I guess the OP expects `%U` not to be ignored.

~~~diff
diff --git a/lib/time.rb b/lib/time.rb
index 3728fef..ec45767 100644
--- a/lib/time.rb
+++ b/lib/time.rb
@@ -194,13 +194,18 @@ class Time

LeapYearMonthDays = [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31] # :nodoc:
CommonYearMonthDays = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31] # :nodoc:
-    def month_days(y, m)
+    def month_days_of_year(y)
if ((y % 4 == 0) && (y % 100 != 0)) || (y % 400 == 0)
-        LeapYearMonthDays[m-1]
+        LeapYearMonthDays
else
-        CommonYearMonthDays[m-1]
+        CommonYearMonthDays
end
end
+    private :month_days_of_year
+
+    def month_days(y, m)
+      month_days_of_year(y)[m-1]
+    end
private :month_days

def apply_offset(year, mon, day, hour, min, sec, off)
@@ -432,7 +437,34 @@ class Time
else
year = d[:year]
year = yield(year) if year && block_given?
-        t = make_time(date, year, d[:mon], d[:mday], d[:hour], d[:min], d[:sec], d[:sec_fraction], d[:zone], now)
+        mon = d[:mon]
+        mday = d[:mday]
+        unless mon and mday
+          case
+          when (w = d[:wnum0])  # %U
+            range = 0..53
+            mday = +1
+          when (w = d[:wnum1])  # %W
+            range = 0..53
+            mday = +2
+          when (w = d[:cweek])  # %V
+            range = 1..53
+            mday = -6
+          end
+          if mday
+            raise ArgumentError, "invalid date" unless range.cover?(w)
+            t = make_time(date, year, 1, 1, nil, nil, nil, nil, d[:zone], now)
+            mday += w * 7 - t.wday
+            if w = d[:wday] || d[:cwday]
+              mday += w
+            end
+            month_days_of_year(t.year).each_with_index do |n, m|
+              break mon = m+1 if mday <= n
+              mday -= n
+            end
+          end
+        end
+        t = make_time(date, year, mon, mday, d[:hour], d[:min], d[:sec], d[:sec_fraction], d[:zone], now)
end
t
end
~~~

----------------------------------------
Bug #9836: Bad Implementation of Time.strptime
https://bugs.ruby-lang.org/issues/9836#change-46720

* Author: Max Anselm
* Status: Feedback
* Priority: Normal
* Assignee:
* Category:
* Target version:
* ruby -v: ruby 2.1.2p95 (2014-05-08 revision 45877) [x86_64-linux]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
----------------------------------------
According to the documentation, `Time.strptime` "parses +date+ using `Date._strptime` and converts it to a Time object." However this conversion is extremely naive, using only certain fields return by `Date._strptime` and ignoring the rest (for example `:wnum0`).

This creates confusing and inconsistent behavior when compared to `Date` and `DateTime`'s `strptime` methods.

For example:
```
puts Date.strptime('201418', "%Y%U")
=> 2014-05-04
puts DateTime.strptime('201418', "%Y%U")
=> 2014-05-04T00:00:00+00:00
puts Time.strptime('201418', "%Y%U")
=> 2014-01-01 00:00:00 -0500
```

--
https://bugs.ruby-lang.org/