Grammatically, the term functions as a noun phrase. The core of the phrase is the compound noun "half birthday." The proper noun phrase "September 11th" acts as an adjectival modifier, specifying the anchor date from which the half birthday is to be calculated.
A half birthday is a date that falls precisely six months before or after an individual's actual date of birth. To determine the date associated with the term, one adds six months to the given date. Adding six calendar months to September 11th results in the date March 11th. This calculation establishes March 11th as the specific date being identified.
The practical application of this term is to label and identify the calendar date, March 11th, in relation to its six-month interval from September 11th. It is used for informal commemorations or personal milestone tracking, deriving its significance from the individual's connection to the initial date. The phrase serves as a specific descriptor for this temporal midpoint.
