Intervals are intervals: they are 'irrespective' with dates. In TD (and in other RDBMS's, like Oracle) if you add (or substract) intervals to dates (as in your example) the result will be pure arithmetic, which can yield an error.
More of this here (in spanish, as usual):
http://carlosal.wordpress.com/2011/06/13/intervals-year-month-y-dates-en...
HTH.
Cheers.
Carlos.
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