Description:
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on 05-31 run strtotime('-1 month') and strtotime('-2 month') the month is not sequential

Test script:
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echo date('Y-m-d H:i:s', strtotime('-1 month')) . "\n";
echo date('Y-m-d H:i:s', strtotime('-2 month')) . "\n";

Expected result:
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the month should be sequential

Actual result:
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-1 month for 05
and -2 month for 03