Python Pandas - Indicate whether the date in DateTimeIndex is the first day of the year

To check whether the date in DateTimeIndex is the first day of the year, use the DateTimeIndex.is_year_start property. This property returns a boolean array indicating which dates are January 1st.

Syntax

DateTimeIndex.is_year_start

Creating a DateTimeIndex

First, let's create a DateTimeIndex that spans across a year boundary to demonstrate the property ?

import pandas as pd

# Create DateTimeIndex spanning year boundary
datetimeindex = pd.date_range('2021-12-30 02:30:50', periods=6, tz='Australia/Adelaide', freq='1D')

print("DateTimeIndex...")
print(datetimeindex)

DateTimeIndex...
DatetimeIndex(['2021-12-30 02:30:50+10:30', '2021-12-31 02:30:50+10:30',
               '2022-01-01 02:30:50+10:30', '2022-01-02 02:30:50+10:30',
               '2022-01-03 02:30:50+10:30', '2022-01-04 02:30:50+10:30'],
              dtype='datetime64[ns, Australia/Adelaide]', freq='D')

Using is_year_start Property

Now let's check which dates are the first day of the year ?

import pandas as pd

datetimeindex = pd.date_range('2021-12-30 02:30:50', periods=6, tz='Australia/Adelaide', freq='1D')

# Check which dates are first day of year
year_start_check = datetimeindex.is_year_start
print("Is year start check:")
print(year_start_check)

# Show corresponding dates
print("\nDates and their year start status:")
for date, is_start in zip(datetimeindex, year_start_check):
    print(f"{date.strftime('%Y-%m-%d')}: {is_start}")

Is year start check:
[False False  True False False False]

Dates and their year start status:
2021-12-30: False
2021-12-31: False
2022-01-01: True
2022-01-02: False
2022-01-03: False
2022-01-04: False

Multiple Year Start Example

Let's create an example with multiple year starts to see the pattern ?

import pandas as pd

# Create index with multiple January 1st dates
dates = ['2020-01-01', '2020-12-31', '2021-01-01', '2021-06-15', '2022-01-01']
datetimeindex = pd.DatetimeIndex(dates)

print("DateTimeIndex:")
print(datetimeindex)

print("\nYear start check:")
print(datetimeindex.is_year_start)

DateTimeIndex:
DatetimeIndex(['2020-01-01', '2020-12-31', '2021-01-01', '2021-06-15',
               '2022-01-01'],
              dtype='datetime64[ns]', freq=None)

Year start check:
[ True False  True False  True]

Return Value

The is_year_start property returns a numpy array of boolean values where:

• True indicates the date is January 1st (first day of the year)
• False indicates the date is not January 1st

Conclusion

The is_year_start property provides a quick way to identify January 1st dates in a DateTimeIndex. It returns a boolean array that can be used for filtering or conditional operations on year-start dates.