Calendar Month vs Fixed Days Comparator

Worked example

From January 31, 2027, adding 1 calendar month constrains to February 28, February's own last day that year. Adding 30 fixed days instead lands on March 2, a full 2 days later. Adding 4 weeks, exactly 28 days, also lands on February 28, coincidentally matching the calendar-month result this particular time, though it will not always agree, since 4 weeks is always exactly 28 days while a calendar month varies from 28 to 31 depending on which month is crossed.

Common mistakes

• Treating "a month," "30 days," and "4 weeks" as interchangeable phrases. All three genuinely diverge as soon as the crossed month is not exactly 30 days long.
• Assuming a recurring "monthly" schedule and an "every 30 days" schedule track together over a year. The monthly version stays anchored to roughly the same day each month, while the 30-day version drifts steadily across the calendar.

Why this matters

A recurring deadline described as "monthly" behaves very differently from one described as "every 30 days" over the course of a year. The monthly version mostly stays anchored to the same day of the month, while the 30-day version drifts across the calendar, since most months are not exactly 30 days long.

Limitations

This compares three specific interpretations side by side. It does not determine which one a specific contract or policy actually means by "a month," so that determination still needs to come from the governing document itself.