Finding day of the week for a datetime64

Question

Apologies if this is an easy one but I can't see anything in the numpy documentation. I have a datetime64 and I'd like to find out which day of the week it is.

Unlike python datetime, datetime64 doesn't seem to have a .weekday() function. However it does have a busday_offset() function, which implies it must know behind-the-scenes what day of the week a given date is. Any help greatly appreciated.

Answer 1

Please look at the following related SO question:

Get weekday/day-of-week for Datetime column of DataFrame

I understand that your data is a numpy array, it may be useful to convert your data into a dataframe format via pandas. This is shown below.

import pandas as pd
df = pd.DataFrame(numpyArray)

Provided your data is now in a dataframe, then the below code (taken from the above link), should provide you with the weekday you are after.

df['weekday'] = df['Timestamp'].dt.dayofweek

Additionally this reference may also assist you.

https://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.dt.dayofweek.html

Answer 2

If you're looking for a genuinely vectorized numpy approach, you could do:

def day_of_week_num(dts):
    return (dts.astype('datetime64[D]').view('int64') - 4) % 7

It's a bit hacky, and just takes advantage of the fact that numpy.datetime64s are relative to the unix epoch, which was a Thursday.

(I don't know if this is an implementational detail that could change without notice, but you can always check with assert np.zeros(1).astype('datetime64[D]') == np.datetime64('1970-01-01', 'D').)