Description:
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Pluralization seems to remove 1 hour from calculation or drops daylight savings adjustment.

Notes:
date.timezone = America/Los_Angeles
Correction to PHP version: 5.3.1

Test script:
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date("Y-m-d H:i:s",strtotime("4 Sunday ago"));
date("Y-m-d H:i:s",strtotime("4 Sundays ago"));

Expected result:
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2010-05-01 00:00:00
2010-05-01 00:00:00

Actual result:
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2010-05-01 00:00:00
2010-04-30 23:00:00