How to Calculate Months Since a Date
Calculating months since a date is useful for tracking early relationship milestones, pregnancy weeks, and sub-annual anniversary intervals. The total months between two dates is a combination of complete years converted to months, plus remaining months, adjusted for the day-of-month comparison. See the Date Calculation for the complete year-by-year breakdown.
Formula: Total Months Between Two Dates
Total months = (Year B − Year A) × 12 + (Month B − Month A) + adjustment. Adjustment: If Day B < Day A, subtract 1 (the current month is not yet complete). Example: Start date March 15, 2024. Today September 10, 2024. Months = (0 × 12) + (9 − 3) + (−1 because day 10 < day 15) = 5 complete months.
Monthly Anniversary Tracking for Early Relationships
Monthly anniversaries are commonly tracked in the first year of a relationship, before the first annual anniversary. Common monthly marks: 1 month (initial milestone), 3 months (quarter-year mark), 6 months (half-year mark). After the first annual anniversary, monthly tracking typically yields to annual tracking. For comparison, explore Years Between Dates.
Month Count Algorithm: Handling Variable Month Lengths
The month count algorithm must handle months of different lengths (28, 29, 30, or 31 days). The formula (Year_B − Year_A) × 12 + (Month_B − Month_A) with day-of-month adjustment correctly handles all cases. Example: a couple married January 31, 2024 — on February 28, 2024, they have completed 0 full months (day 28 < day 31). On March 1, 2024, they have completed 1 full month (day 1 < day 31 → subtract 1 → 2−1=1 month). The day adjustment is critical for dates near the end of the month. Related reading: Days Since Wedding.
Months Calculation — Technical Reference
- ▸Formula: (Year_B - Year_A) × 12 + (Month_B - Month_A) - (1 if Day_B < Day_A)
- â–¸JavaScript: differenceInMonths() from date-fns
- ▸Note: months have varying lengths — day-of-month comparison is essential