WhenMathCalendar Month vs Fixed Days Comparator
"A month," "30 days," and "4 weeks" sound interchangeable but usually aren't. Enter a start date and see all three land on their actual, sometimes different, dates.
Worked example
From January 31, 2027: adding 1 calendar month constrains to February 28 (February's last day that year). Adding 30 fixed days lands on March 2. Adding 4 weeks (28 days) also lands on February 28, coincidentally matching the calendar-month result this time, though it will not always agree, since 4 weeks is always exactly 28 days while a calendar month varies from 28 to 31.
Why this matters
A recurring deadline described as "monthly" behaves very differently from one described as "every 30 days" over a year: the monthly version stays anchored to the same day of the month (mostly), while the 30-day version drifts across the calendar since most months aren't exactly 30 days.
Limitations
This compares three specific interpretations; it does not determine which one a specific contract or policy means by "a month."
Frequently asked questions
- Will these three always disagree?
- No. They often agree in months with exactly 28 days, but diverge in longer months, which is exactly the ambiguity this page exists to surface.