Description:
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strtotime() outputs the xth + 1 weekday instead of the xth weekday when the defined month starts on the defined weekday.

Example: The expected output of strtotime('January 2018 third Monday') is the timestamp for '2018-01-15', which is the third Monday of January 2018. Instead, strtotime() outputs the timestamp for '2018-01-22', which is the fourth Monday of January 2018 because that month starts on a Monday.

Test script:
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print 'Expected: 2018-01-15. Result: ' . date('Y-m-d', strtotime('january 2018 third monday'));