The International Month Calendar (IMC) is a subset of the International Calendar (IC), hence a standardized variant of the proleptic Gregorian calendar (PGC). The International Standard Month Calendar (ISMC) is a restricted subset of the IMC. It runs in parallel with the International Week Calendar (IWC) and the International Day Calendar (IDC).
The month year has 365 days in a common year or 366 days in a leap year. It’s divided into 12 months of unsystematic length: 31, 28 (or 29 with leap day), 31, 30, 31, 30, 31, 31, 30, 31, 30 and 31 days. Three consecutive months make one of four triads. They are 90 (or 91 with leap day), 91, 92 and 92 days long, respectively.
Weeks are not used with the IMC. The date notation specified in ISO 8601 / ISC and IC is commonly used for the IMC, but is not a mandatory part of it.