Description:
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The logical behaviour of a DateInterval created with the "next" keyword, when subtracted from a DateTime, should be to use "last" instead of "next" in the calculation (and vice versa).

Instead, it appears that DateIntervals created with "next" or "last" keywords cannot be subtracted from DateTimes with any accuracy. A DateTime results from the operation but has an unexpected value.

Reproduce code:
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//positive DateInterval
$da1=date_create();
$ds1=date_create();
$i=DateInterval::createFromDateString('third Tuesday of next month');
echo $da1->format('Y-m-d');
echo date_add($da1,$i)->format('Y-m-d');//works
echo date_sub($ds1,$i)->format('Y-m-d');//fails with weird date

//negative DateInterval
$da2=date_create();
$ds2=date_create();
$i2=DateInterval::createFromDateString('third Tuesday of last month');
echo $da2->format('Y-m-d');
echo date_add($da2,$i2)->format('Y-m-d');//works
echo date_sub($ds2,$i2)->format('Y-m-d');//fails with weird date

Expected result:
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2009-11-02 //today
2009-12-15 //third Tuesday of December
2009-10-20 //third Tuesday of October

2009-11-02 //today
2009-10-20 //third Tuesday of October
2009-12-15 //third Tuesday of December

Actual result:
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2009-11-02 //today
2009-12-15 //third Tuesday of December
2009-09-18 //third Friday of September

2009-11-02 //today
2009-10-20 //third Tuesday of October
2009-11-18 //third Wednesday of November