Physical presence refers to the number of days the applicant must physically be present in the United States during the statutory period up to the date of filing for naturalization.
This does not say up to the date of interview or oath ceremony. So as long as I am under 18 months by the time I file, I am good. To my understanding, I should have no problem if by the time of my interview I am a few days over the 18 months.
Physical presence is calculated up to the oath ceremony. If you fall below the physical presence requirement prior to the oath ceremony your petition will be denied and you will need to re-apply. Furthermore trips of any duration can also impact continuous residency depending on the nature of the trip.
If you are unsure about the requirements for naturalization I strongly recommend using an attorney to assist you with your petition.
Representing clients throughout the US and around the world: www.PogueImmigrationLaw.com (513) 549-4420. We cannot provide legal advice or recommendations unless you retain our law firm to represent you. No attorney/client relationship will begin until you sign a representation agreement and make a retainer payment to open a case with us. Any information found here is general in nature and should not be relied upon in individual cases.
INA 316(a)(1) states in pertinent part "and during the five years immediately preceding the date of filing his application has been physically present therein for periods totaling at least half of that time." The same language appears in INA 319(a) The words filing his application support your assertion. Whether or not you meet that test is up to your fact checking and USCIS's determination. You also must maintain continuous residence and be a person of good moral character.