Numpy and Pandas date arithmetic

numpy_date_arithmetic.py

import pandas as pd
import numpy as np

#  
td = pd.DataFrame({'Date': pd.date_range('2014-01-01', '2015-12-31')})
td['Timedelta'] = td['Date'].max() - td['Date']

td.dtypes
# Date          datetime64[ns]
# Timedelta    timedelta64[ns]
# dtype: object

td["AgeDays"] = td["Timedelta"] / np.timedelta64(1, 'D')
td["AgeMonths"] = td["Timedelta"] / np.timedelta64(1, 'M')
td["AgeYears"] = td["Timedelta"] / np.timedelta64(1, 'Y')

td.head()

#         Date  Timedelta  AgeDays  AgeMonths  AgeYears
# 0 2014-01-01   729 days      729  23.951210  1.995934
# 1 2014-01-02   728 days      728  23.918356  1.993196
# 2 2014-01-03   727 days      727  23.885501  1.990458
# 3 2014-01-04   726 days      726  23.852646  1.987720
# 4 2014-01-05   725 days      725  23.819791  1.984983


# Days in a month
(td["AgeDays"] / td["AgeMonths"]).head()
# 0    30.436875
# 1    30.436875

# Days in a Year
(td["AgeDays"] / td["AgeYears"]).head()
# 0    365.2425
# 1    365.2425

## PERIODS

td["Month"] = td["Date"].dt.to_period("M")
td["Year"] = td["Date"].dt.to_period("Y")

td.head()
#         Date  Timedelta  AgeDays    Month  Year
# 0 2014-01-01   729 days      729  2014-01  2014
# 1 2014-01-02   728 days      728  2014-01  2014


## TYPES
td.dtypes

# Date          datetime64[ns]
# Timedelta    timedelta64[ns]
# AgeDays              float64
# Month                 object
# Year                  object
# dtype: object

d0 = td["Date"].iloc[0]
p0 = td["Month"].iloc[0]

type(d0)
# pandas.tslib.Timestamp 
type(p0)
# pandas._period.Period


# Work out the first date of the week or first date of the Month
week_start = pd.offsets.Week(weekday=0)
month_begin = pd.offsets.MonthBegin()

td["WeekStart"] = td.Date.apply(lambda x: week_start.rollback(x))
td["MonthStart"] = td.Date.apply(lambda x: month_begin.rollback(x))

print(td[["Date", "MonthStart", "WeekStart"]].head(25))

#         Date MonthStart  WeekStart
# 0  2014-01-01 2014-01-01 2013-12-30
# 1  2014-01-02 2014-01-01 2013-12-30
# 2  2014-01-03 2014-01-01 2013-12-30
# 3  2014-01-04 2014-01-01 2013-12-30
# 4  2014-01-05 2014-01-01 2013-12-30
# 5  2014-01-06 2014-01-01 2014-01-06
# 6  2014-01-07 2014-01-01 2014-01-06
# 7  2014-01-08 2014-01-01 2014-01-06
# 8  2014-01-09 2014-01-01 2014-01-06
# 9  2014-01-10 2014-01-01 2014-01-06
# 10 2014-01-11 2014-01-01 2014-01-06
# 11 2014-01-12 2014-01-01 2014-01-06
# 12 2014-01-13 2014-01-01 2014-01-13
# 13 2014-01-14 2014-01-01 2014-01-13
# 14 2014-01-15 2014-01-01 2014-01-13
# 15 2014-01-16 2014-01-01 2014-01-13
# 16 2014-01-17 2014-01-01 2014-01-13
# 17 2014-01-18 2014-01-01 2014-01-13
# 18 2014-01-19 2014-01-01 2014-01-13
# 19 2014-01-20 2014-01-01 2014-01-20
# 20 2014-01-21 2014-01-01 2014-01-20
# 21 2014-01-22 2014-01-01 2014-01-20
# 22 2014-01-23 2014-01-01 2014-01-20
# 23 2014-01-24 2014-01-01 2014-01-20
# 24 2014-01-25 2014-01-01 2014-01-20