The 360-day calendar is a method of measuring durations used in financial markets, in computer models, in ancient literature, and in prophetic literary genres. It is based on merging the three major calendar systems into one complex clock, with the 360-day year as the average year of the lunar and the solar: [365.2425 (solar) + 354.3829 (lunar)] = 719.61 ÷ 2 = 359.8 days rounded to 360. It is a simplification to a 360-day year, consisting of 12 months of 30 days each. To derive such a calendar from the standard Gregorian calendar, certain days are skipped.
Ancient calendars around the world initially used a 360 day calendar.
Romans initially used a calendar which had 360 days, with varying length of months.
The Rig Veda describes a calendar with twelve months and 360 days.
In the Mayan Long Count Calendar, the equivalent of the year, the tun, was 360 days.
A duration is calculated as an integral number of days between start date A and end date B. The difference in years, months and days are usually calculated separately:
d
u
r
a
t
i
o
n
(
A
,
B
)
=
(
B
y
−
A
y
)
×
360
+
(
B
m
−
A
m
)
×
30
+
(
B
d
−
A
d
)
;
A
≤
B
There are several methods commonly available which differ in the way that they handle the cases where the months are not 30 days long, i.e. how they adjust dates:
European method (30E/360)
If either date A or B falls on the 31st of the month, that date will be changed to the 30th.
Where date B falls on the last day of February, the actual date B will be used.
All months are considered to last 30 days and hence a full year has 360 days.
US/NASD method (30US/360)
If both date A and B fall on the last day of February, then date B will be changed to the 30th.
If date A falls on the 31st of a month or last day of February, then date A will be changed to the 30th.
If date A falls on the 30th of a month after applying (2) above and date B falls on the 31st of a month, then date B will be changed to the 30th.
All months are considered to last 30 days and hence a full year has 360 days.
ISDA method
If date A falls on the 31st of a month, then date A will be changed to the 30th.
If date A falls on the 30th of the month after applying the rule above, and date B falls on the 31st of the month, then date B will be changed to the 30th.
All months are considered to last 30 days except February which has its actual length. Any full year, however, always counts for 360 days.
BMA/PSA method
If date A falls on the 31st of a month or last day of February, then date A will be changed to the 30th.
If date A falls on the 30th of the month after applying the rule above, and date B falls on the 31st of the month, then date B will be changed to the 30th.
All months are considered to last 30 days and hence a full year has 360 days.
Alternative European method (30E+/360)
If date A falls on the 31st of a month, then date A will be changed to the 30th.
If date B falls on the 31st of a month, then date B will be changed to the 1st of the following month.
Where date B falls on the last day of February, the actual date B will be used.
All months are considered to last 30 days and hence a full year has 360 days.
