numpy and pandas timedelta error

Question

In Python I have an array of dates generated (or read from a CSV-file) using pandas, and I want to add one year to each date. I can get it working using pandas but not using numpy. What am I doing wrong? Or is it a bug in either pandas or numpy?

Thanks!

import numpy as np
import pandas as pd
from pandas.tseries.offsets import DateOffset

# Generate range of dates using pandas.
dates = pd.date_range('1980-01-01', '2015-01-01')

# Add one year using pandas.
dates2 = dates + DateOffset(years=1)

# Convert result to numpy. THIS WORKS!
dates2_np = dates2.values

# Convert original dates to numpy array.
dates_np = dates.values

# Add one year using numpy. THIS FAILS!
dates3 = dates_np + np.timedelta64(1, 'Y')

# TypeError: Cannot get a common metadata divisor for NumPy datetime metadata [ns] and [Y] because they have incompatible nonlinear base time units

Answer 1

Adding np.timedelta64(1, 'Y') to an array of dtype datetime64[ns] does not work because a year does not correspond to a fixed number of nanoseconds. Sometimes a year is 365 days, sometimes 366 days, sometimes there is even an extra leap second. (Note extra leap seconds, such as the one that occurred on 2015-06-30 23:59:60, are not representable as NumPy datetime64s.)

The easiest way I know to add a year to a NumPy datetime64[ns] array is to break it into constituent parts, such as years, months and days, do the computation on integer arrays, and then recompose the datetime64 array:

def year(dates):
    "Return an array of the years given an array of datetime64s"
    return dates.astype('M8[Y]').astype('i8') + 1970

def month(dates):
    "Return an array of the months given an array of datetime64s"
    return dates.astype('M8[M]').astype('i8') % 12 + 1

def day(dates):
    "Return an array of the days of the month given an array of datetime64s"
    return (dates - dates.astype('M8[M]')) / np.timedelta64(1, 'D') + 1

def combine64(years, months=1, days=1, weeks=None, hours=None, minutes=None,
              seconds=None, milliseconds=None, microseconds=None, nanoseconds=None):
    years = np.asarray(years) - 1970
    months = np.asarray(months) - 1
    days = np.asarray(days) - 1
    types = ('<M8[Y]', '<m8[M]', '<m8[D]', '<m8[W]', '<m8[h]',
             '<m8[m]', '<m8[s]', '<m8[ms]', '<m8[us]', '<m8[ns]')
    vals = (years, months, days, weeks, hours, minutes, seconds,
            milliseconds, microseconds, nanoseconds)
    return sum(np.asarray(v, dtype=t) for t, v in zip(types, vals)
               if v is not None)

# break the datetime64 array into constituent parts
years, months, days = [f(dates_np) for f in (year, month, day)]
# recompose the datetime64 array after adding 1 to the years
dates3 = combine64(years+1, months, days)

yields

In [185]: dates3
Out[185]: 
array(['1981-01-01', '1981-01-02', '1981-01-03', ..., '2015-12-30',
       '2015-12-31', '2016-01-01'], dtype='datetime64[D]')

Despite appearing to be so much code, it is actually quicker than adding a DateOffset of 1 year:

In [206]: %timeit dates + DateOffset(years=1)
1 loops, best of 3: 285 ms per loop

In [207]: %%timeit
   .....: years, months, days = [f(dates_np) for f in (year, month, day)]
   .....: combine64(years+1, months, days)
   .....: 
100 loops, best of 3: 2.65 ms per loop

Of course, pd.tseries.offsets offers a whole panoply of offsets that have no easy counterpart when working with NumPy datetime64s.

Answer 2

Here is what it says in the numpy documentation:

There are two Timedelta units (‘Y’, years and ‘M’, months) which are treated specially, because how much time they represent changes depending on when they are used. While a timedelta day unit is equivalent to 24 hours, there is no way to convert a month unit into days, because different months have different numbers of days.

Days and weeks seem to work though:

dates4 = dates_np + np.timedelta64(1, 'D')
dates5 = dates_np + np.timedelta64(1, 'W')