This D-Day calculator handles every kind of date counting scenario—days remaining until university entrance exams and certification tests, wedding anniversaries and couple milestones (100 days, 1-year), baby milestone events (100-day Baekil, 200-day, first birthday Doljanchi), travel departure countdowns, and project deadlines. Two counting modes are supported: 'anniversary mode' (start day = Day 1, moving forward as D+1) commonly used for couple milestones and baby milestones, and 'countdown mode' (full 24-hour remaining days, shown as D-) used for exam preparation and event countdowns. It automatically extracts key milestone dates—100/200/500/1000 days and 1/2/3-year anniversaries—so you never miss a celebration. A companion date-offset tool computes any past or future calendar date by adding or subtracting days, useful for tax filing deadlines, contract expiry planning, and pregnancy due-date estimation. Convention: D-0 (or D-Day) is the event day itself, D-1 is the day before, and D+1 marks the day after.