International Standard Month Calendar

The International Standard Month Calendar (ISMC) is a subset of the International Standard Calendar (ISC), hence a standardized variant of the proleptic Gregorian calendar (PGC). The International Month Calendar (IMC) is an expansion of the ISMC. It runs in parallel with the International Standard Week Calendar (ISWC).

International Standard Month Calendar
Created by	ISO 8601
Week
days	7
day starts	Midnight
Month
days	28–31
Season
days
Year
months	12
31	January
28+	February
31	March
30	April
31	May
30	June
31	July
31	August
30	September
31	October
30	November
31	December
weeks	48
days	365
year starts	1 January (like Gregorian)
leap day at	29 February
Cycle
years	400 ( leap)
seasons	1600
era starts	0000 (like proleptic Gregorian)