How can we do date and time math in Python?

It is very easy to do date and time math in Python using timedelta objects. Whenever you want to add or subtract to a date/time, use a datetime.datetime(), then add or subtract datetime.timedelta() instances. A timedelta object represents a duration, the difference between two dates or times.

Syntax

The timedelta constructor has the following function signature ?

datetime.timedelta([days[, seconds[, microseconds[, milliseconds[, minutes[, hours[, weeks]]]]]]])

Note: All arguments are optional and default to 0. Arguments may be integers, longs, or floats, and may be positive or negative.

Adding and Subtracting Time

An example of using the timedelta objects to subtract time from the current datetime ?

import datetime

old_time = datetime.datetime.now()
print("Current time:", old_time)

new_time = old_time - datetime.timedelta(hours=2, minutes=10)
print("Time 2 hours 10 minutes ago:", new_time)

Current time: 2024-01-04 11:09:00.694602
Time 2 hours 10 minutes ago: 2024-01-04 08:59:00.694602

Adding Time to Current Date

You can also add time using positive values in timedelta ?

import datetime

current_time = datetime.datetime.now()
print("Current time:", current_time)

future_time = current_time + datetime.timedelta(days=7, hours=3)
print("7 days and 3 hours later:", future_time)

Current time: 2024-01-04 11:09:00.694602
7 days and 3 hours later: 2024-01-11 14:09:00.694602

Calculating Difference Between Dates

Subtracting two datetime objects gives a timedelta object. This timedelta object can be used to find the exact difference between the two datetimes ?

import datetime
import time

t1 = datetime.datetime.now()
time.sleep(2)  # Wait 2 seconds
t2 = datetime.datetime.now()

difference = t2 - t1
print("Time difference:", difference)
print("Type of difference:", type(difference))
print("Difference in seconds:", difference.total_seconds())

Time difference: 0:00:02.003456
Type of difference: <class 'datetime.timedelta'>
Difference in seconds: 2.003456

Working with Time Objects

timedelta() arithmetic is not supported for datetime.time() objects. If you need to use offsets from an existing datetime.time() object, use datetime.datetime.combine() to form a datetime.datetime() instance, do your calculations, and extract the time again with the .time() method ?

import datetime

# Create a time object
time_obj = datetime.time(14, 30, 0)  # 2:30 PM
today = datetime.date.today()

# Combine with today's date
datetime_obj = datetime.datetime.combine(today, time_obj)
print("Original datetime:", datetime_obj)

# Add 2 hours
new_datetime = datetime_obj + datetime.timedelta(hours=2)
print("After adding 2 hours:", new_datetime)

# Extract just the time
new_time = new_datetime.time()
print("New time only:", new_time)

Original datetime: 2024-01-04 14:30:00
After adding 2 hours: 2024-01-04 16:30:00
New time only: 16:30:00

Conclusion

Python's datetime.timedelta makes date and time arithmetic simple and intuitive. Use it to add or subtract time periods from datetime objects, and remember to convert time objects to datetime objects before performing arithmetic operations.