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diff --git a/challenge-187/sgreen/python/ch-1.py b/challenge-187/sgreen/python/ch-1.py
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+++ b/challenge-187/sgreen/python/ch-1.py
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+#!/usr/bin/env python3
+
+import sys
+from datetime import date
+
+
+def get_dofy(dt):
+    '''Turn a date in DD-MM format into day of year'''
+    (day, month) = dt.split('-')
+    d = date(2022, int(month), int(day))
+    return int(d.strftime('%j'))
+
+
+def main(array):
+    '''Calculate the days that foo and bar spent together'''
+
+    # Convert the input into the day of year
+    foo_start = get_dofy(array[0])
+    foo_end = get_dofy(array[1])
+    bar_start = get_dofy(array[2])
+    bar_end = get_dofy(array[3])
+
+    # Make sure they arrive before they leave!
+    if foo_end < foo_start:
+        raise ValueError('foo left before they arrived!')
+    if bar_end < bar_start:
+        raise ValueError('bar left before they arrived!')
+
+    # The time they spent together is when the last person arrives and the first one leave.
+    both_start = max(foo_start, bar_start)
+    both_end = min(foo_end, bar_end)
+
+    # Display the days spent together
+    if both_end < both_start:
+        print('0 days')
+    elif both_start == both_end:
+        print('1 day')
+    else:
+        print(f'{both_end - both_start + 1} days')
+
+
+if __name__ == '__main__':
+    main(sys.argv[1:5])