days_in_month

days_in_month#

Overview#

NCL’s days_in_month calculates the number of days in a month given a month and year.

Grab and Go#

import cftime

day = 1
month = 2
year = 2024

days = cftime.datetime(year, month, day, calendar='standard').daysinmonth

days

29

Using the calendar module#

The calendar module is part of the Python Standard Library and could be sufficient to calculate the number of days in a month.

from calendar import monthrange

month = 2
year = 1500
first_day, num_days = monthrange(year, month)
print(f"Month {month} in {year} has {num_days} days")

Month 2 in 1500 has 28 days

However, the calendar only uses a proleptic Gregorian calendar. If you need to work with other calendars, the calendar module will not be sufficient.

Using cftime#

cftime supports all of the CF Convention calendars. Here’s how you can use cftime to calculate the number of days in a month.

import cftime

day = 1
month = 2
year = 1500

days = cftime.datetime(year, month, day, calendar='standard').daysinmonth
print(f"standard calendar: \t\t\t\t month {month} in {year} has {days} days")

days = cftime.datetime(year, month, day, calendar='proleptic_gregorian').daysinmonth
print(f"proleptic Gregorian calendar: \t month {month} in {year} has {days} days")

days = cftime.datetime(year, month, day, calendar='julian').daysinmonth
print(f"Julian calendar: \t\t\t\t month {month} in {year} has {days} days")

days = cftime.datetime(year, month, day, calendar='360_day').daysinmonth
print(f"360 day calendar: \t\t\t\t month {month} in {year} has {days} days")

standard calendar: 				 month 2 in 1500 has 29 days
proleptic Gregorian calendar: 	 month 2 in 1500 has 28 days
Julian calendar: 				 month 2 in 1500 has 29 days
360 day calendar: 				 month 2 in 1500 has 30 days

Differences between cftime and NCL’s days_in_month#

Calendars#

The calendars available in cftime and NCL’s days_in_month, while both based on the CF Conventions calendars, are slightly different.

  • cftime does support using "none" as a calendar type, but not for this particular function.

  • NCL’s days_in_month does not support the proleptic_gregorian calendar.

Numerical Differences#

Additionally, the "standard" and "gregorian" calendars seem to disagree for some historical dates.

Input type#

Notably, using cftime to calculate the number of days in a month works by getting a daysinmonth value from a cftime.datetime object, which means that the calculations for days in a month have to be collected through each date individually, while NCL’s days_in_month can take in multidimensional integer arrays, given that the year and month arrays have the same dimensions.

Python Resources#

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