Appraisal Reference

Months Elapsed

Counting months between two dates sounds like the least interesting thing on this site. It is also the input to the market conditions adjustment, which is the first adjustment in the sales comparison sequence and the one most often waved at rather than measured.

The tool

Pick two dates. The same widget appears inside the Sales Comparison problems that need it.

Typically the first date is the comparable's date of sale and the second is the effective date of your appraisal. Reverse them and the tool says so rather than handing back a negative.

What it counts — and what it ignores

Whole calendar months. Days are not part of the calculation.

months = (year₂ − year₁) × 12 + (month₂ − month₁)

That is the whole formula, and it deliberately takes no notice of the day of the month. A sale on 31 January and an effective date of 1 February read as one month apart, even though a single day separates them. A sale on 1 January and an effective date of 31 January read as zero.

This is the ordinary appraisal treatment, not a shortcut. Market conditions are extracted from paired sales measured in whole months, and applying a rate developed that way to a count made in days would be mixing two different measures. The convention is stated here rather than left to be discovered, because a student checking the arithmetic by hand will otherwise conclude the tool is broken.

Sale date	Effective date	Months	Note
January 2023	July 2024	18	1 year, 6 months
December 2022	January 2025	25	crosses two year boundaries
March 2024	March 2024	0	same month, no adjustment
June 2024	January 2024	—	effective date precedes the sale

What the number is for

A month count on its own adjusts nothing. It is one of two figures.

The other is a rate per month, extracted from the market — usually from resales of the same property, or from paired sales that differ only in when they sold. Multiply the two and you have the adjustment.

Months elapsed Rate per month %

Method	Total adjustment	On a $300,000 sale
Straight line
Compounded
Difference

Over a few months the two are almost the same. Over eighteen or twenty-four they part company by enough to matter, and over three years the gap is not defensible as a rounding difference.

The way you extract it is the way you apply it. If the rate came out of paired resales measured on a straight-line basis, apply it straight-line. If it was derived by compounding, compound it. Extracting one way and applying the other produces an error that no amount of careful arithmetic will catch, because every individual calculation is correct.

Market conditions is the first adjustment, and it is the one whose order matters. It is a percentage, and the adjustments around it are often dollar amounts — expenditures made immediately after purchase, for instance. A dollar amount applied before a percentage is carried through it; applied after, it is not. That is why the sequence of adjustments is prescribed rather than left to preference.