Python program to find number of days between two given dates

Given two dates, Write a Python program to find the total number of days between them. 

Examples:

Input : dt1 = 13/12/2018, dt2 = 25/2/2019
Output : 74 days

Input : dt1 = 01/01/2004, dt2 = 01/01/2005
Output : 366 days 

Naive Approach: One Naive Solution is to start from dt1 and keep counting the days till dt2 is reached. This solution requires more than O(1) time. 

A Better and Simple solution is to count the total number of days before dt1 from i.e., total days from 00/00/0000 to dt1, then count the total number of days before dt2. Finally, return the difference between the two counts.  

Python3

# Python3 program two find number of
# days between two given dates

# A date has day 'd', month 'm' and year 'y'


class Date:
    def __init__(self, d, m, y):
        self.d = d
        self.m = m
        self.y = y


# To store number of days in all months from
# January to Dec.
monthDays = [31, 28, 31, 30, 31, 30,
             31, 31, 30, 31, 30, 31]

# This function counts number of leap years
# before the given date


def countLeapYears(d):

    years = d.y

    # Check if the current year needs to be considered
    # for the count of leap years or not
    if (d.m <= 2):
        years -= 1

    # An year is a leap year if it is a multiple of 4,
    # multiple of 400 and not a multiple of 100.
    return int(years / 4) - int(years / 100) + int(years / 400)


# This function returns number of days between two
# given dates
def getDifference(dt1, dt2):

    # COUNT TOTAL NUMBER OF DAYS BEFORE FIRST DATE 'dt1'

    # initialize count using years and day
    n1 = dt1.y * 365 + dt1.d

    # Add days for months in given date
    for i in range(0, dt1.m - 1):
        n1 += monthDays[i]

    # Since every leap year is of 366 days,
    # Add a day for every leap year
    n1 += countLeapYears(dt1)

    # SIMILARLY, COUNT TOTAL NUMBER OF DAYS BEFORE 'dt2'

    n2 = dt2.y * 365 + dt2.d
    for i in range(0, dt2.m - 1):
        n2 += monthDays[i]
    n2 += countLeapYears(dt2)

    # return difference between two counts
    return (n2 - n1)


# Driver program
dt1 = Date(13, 12, 2018)
dt2 = Date(25, 2, 2019)

print(getDifference(dt1, dt2), "days")

Output

74 days

Time Complexity: O(1)
Auxiliary Space: O(1)

Using Python datetime module:

Python comes with an inbuilt datetime module that helps us to solve various datetime related problems. In order to find the difference between two dates we simply input the two dates with date type and subtract them, which in turn provides us the number of days between the two dates. 

Python3

# Python3 program to find number of days
# between two given dates
from datetime import date


def numOfDays(date1, date2):
  #check which date is greater to avoid days output in -ve number
    if date2 > date1:
        return (date2-date1).days
    else:
        return (date1-date2).days


# Driver program
date1 = date(2018, 12, 13)
date2 = date(2015, 2, 25)
print(numOfDays(date1, date2), "days")

Output

74 days

Time Complexity: O(1)
Auxiliary Space: O(1)

One approach is to use the dateutil library, which provides a more convenient and flexible way of handling dates and times in Python. To install dateutil, you can use the following command:

pip install python-dateutil

To find the number of days between two dates using dateutil, you can use the rdelta function from the relativedelta module, as shown in the example below:

Python3

from dateutil import relativedelta
from datetime import datetime

# Parse the dates from strings into datetime objects
date1 = datetime.strptime("13/10/2018", "%d/%m/%Y")
date2 = datetime.strptime("25/12/2018", "%d/%m/%Y")

# Calculate the difference between the two dates
difference = relativedelta.relativedelta(date2, date1)
# Print the number of days between the two dates
print(difference.months, "months", difference.days, "days")

# This code is contributed by Edula Vinay Kumar Reddy

Time Complexity: O(1)
Auxiliary Space: O(1)

Method#4: Using time module

Approach 

this approach uses the date library in Python to convert the input strings to date objects, then calculates the difference between the two dates using the timedelta function, and finally returns the number of days in the timedelta object.

Algorithm

1. Convert the input strings to timestamp using time.mktime() method
2. Calculate the difference between the two timestamps
3. Convert the difference to days by dividing it with the number of seconds in a day (86400)
4. Return the number of days as integer

Python3

import time


def days_between_dates(dt1, dt2):
    date_format = "%d/%m/%Y"
    a = time.mktime(time.strptime(dt1, date_format))
    b = time.mktime(time.strptime(dt2, date_format))
    delta = b - a
    return int(delta / 86400)


dt1 = "13/12/2018"
dt2 = "25/2/2019"
print(days_between_dates(dt1, dt2))

Output

74

Time Complexity: O(1) (time operations take constant time)
Space Complexity: O(1) (no extra data structures used)

Method#5: Using reduce from functools:

Algorithm:

1. Import the required modules date from datetime and reduce from functools.
2. Define a function numOfDays that takes two arguments date1 and date2.
3. The function uses reduce to apply a lambda function to a list [date1, date2].
4. The lambda function takes two arguments x and y and calculates the number of days between the two dates y-x.
5. The final result is the number of days between date1 and date2.
6. Assign date(2018, 12, 13) to date1 and date(2019, 2, 25) to date2.
7. Call numOfDays function with date1 and date2 as arguments.
8. Print the result along with the string “days”.

Python3

from datetime import date
from functools import reduce


def numOfDays(date1, date2):
    return reduce(lambda x, y: (y-x).days, [date1, date2])


date1 = date(2018, 12, 13)
date2 = date(2019, 2, 25)
print(numOfDays(date1, date2), "days")

# This code is contributed by Jyothi pinjala

Output

74 days

Time complexity: O(n), where n is the number of dates between date1 and date2. In this case, since there are only two dates, the time complexity is constant.
Auxiliary Space: O(1). The function uses a constant amount of memory regardless of the size of the input dates.