Age in Months and Days: Precise, Calendar‑Aware Methods

Some forms and milestones need age beyond whole years. Here’s how to compute age in months and days correctly.

When Precision Matters

• Pediatric growth and developmental milestones
• Medical dosing ranges
• Scholarship or program eligibility with strict cutoffs

Method (High Level)

1. Normalize both dates to local midnight to avoid time‑zone shifts.
2. Compute years by comparing month/day; then months and remaining days.
3. Handle rollovers across month boundaries and leap years consistently.

Inclusive vs Exclusive Rules

• Define whether the end day counts toward age. Many contexts treat age changes at the start of the birthday (exclusive of the day prior).

Consistency Tips

• Use the same calendar library for all computations.
• Write unit tests for tricky cases: end‑of‑month, Feb 29 births, timezone boundaries.

FAQs

Why do tools differ? Libraries pick different inclusive/exclusive conventions and month‑end rules. Choose and document one approach for your use case.