The basic count: end date minus start date
From 14 September 2026 to 31 December 2026, exclusive of the end date, is 108 days — 15 full weeks plus 3 remainder days.
What "including the end date" changes
Counting the same range but including 31 December as one of the days counted gives 109 days instead of 108 — exactly one more, since including the end date always adds exactly 1 to an exclusive count.
Why the distinction matters in practice
Counting nights of a hotel stay is naturally exclusive (check-in to check-out), while counting the total calendar days a rental period covers is often inclusive (both the first and last day belong to the tenant). Getting this wrong by one day is a common source of off-by-one errors in date math.
What this count doesn't tell you
This total-days figure treats every day identically — it doesn't say how many whole years or months are covered, which is a different question answered by a calendar-accurate years/months/days breakdown, covered in the companion article on date difference.