numpy datetime64 add or substract date interval

Question

I am parsing a huge ascii file with dates assigned to entries. So, I found myself using datetime package in parallel to numpy.datetime64 to add array capabilities. I know that the pandas package is probably most recommended to be used for date, however try to pull this off without pandas. I have been looking around for a neat way to add/subtract a certain datestep like one year, or 3 month from a datetime64 object.

Currently, I am converting dt64 object to dt object and use replace function to change the year for example and have to convert it back to dt64 afterward which is a bit messy to me. So, I would appreciate if anyone has a better solution using only numpy.datetime64 format.

Example: Converting a "YYYY-12-31" to "(YYYY-1)-12-31"

a = np.datetime64(2014,12,31)               # a is dt64 object b = a.astype(object)                        # b is dt object converted from a c = np.datetime64( b.replace(b.year-1))     # c is dt64 object shifted back 1 year (a -1year) 

Answer 1

You can use the numpy.timedelta64 object to perform time delta calculations on a numpy.datetime64 object, see Datetime and Timedelta Arithmetic.

Since a year can be either 365 or 366 days, it is not possible to substract a year, but you could substract 365 days instead:

import numpy as np np.datetime64('2014-12-31') - np.timedelta64(365,'D') 

results in:

numpy.datetime64('2013-12-31')