Python program to convert time from 12 hour to 24 hour format

Given a time in 12-hour AM/PM format, convert it to military (24-hour) time. Note : Midnight is 12:00:00 AM on a 12-hour clock and 00:00:00 on a 24-hour clock. Noon is 12:00:00 PM on 12-hour clock and 12:00:00 on 24-hour clock. Examples :

Input : 11:21:30 PM
Output : 23:21:30

Input : 12:12:20 AM
Output : 00:12:20

How to Convert AM/PM to 24 Hour Time

Whether the time format is 12 hours or not, can be found out by using list slicing. Check if last two elements are PM, then simply add 12 to them. If AM, then don’t add. Remove AM/PM from the updated time.   Below is the implementation : 

Python3

# Python program to convert time
# from 12 hour to 24 hour format

# Function to convert the date format
def convert24(str1):

    # Checking if last two elements of time
    # is AM and first two elements are 12
    if str1[-2:] == "AM" and str1[:2] == "12":
        return "00" + str1[2:-2]

    # remove the AM
    elif str1[-2:] == "AM":
        return str1[:-2]

    # Checking if last two elements of time
    # is PM and first two elements are 12
    elif str1[-2:] == "PM" and str1[:2] == "12":
        return str1[:-2]

    else:

        # add 12 to hours and remove PM
        return str(int(str1[:2]) + 12) + str1[2:8]

# Driver Code
print(convert24("08:05:45 PM"))

Output :

20:05:45

Time Complexity: O(1)

Auxiliary Space: O(1)

Here is another approach to the problem that uses the datetime module in Python to convert the time from 12-hour to 24-hour format:

Python3

from datetime import datetime

def convert24(time):
    # Parse the time string into a datetime object
    t = datetime.strptime(time, '%I:%M:%S %p')
    # Format the datetime object into a 24-hour time string
    return t.strftime('%H:%M:%S')

print(convert24('11:21:30 PM'))  # Output: '23:21:30'
print(convert24('12:12:20 AM'))  # Output: '00:12:20'

Output

23:21:30
00:12:20

This approach has the advantage of handling invalid time formats and edge cases, such as the time being in an invalid format or the hours being greater than 12. It also allows for the input time to be in any valid time format recognized by the datetime module, such as using a single digit for the hour or using a different separator between the time parts.

The time complexity of this approach is O(1), as it only involves parsing and formatting the time string.

Using regular expression

Use regular expressions to extract the hour, minute, second, and AM/PM parts from the input time string. Apply the necessary logic to convert the hour to 24-hour format. Format the hour, minute, and second parts into the desired 24-hour time format.

Algorithm

1. Use regular expressions to extract the hour, minute, second, and AM/PM parts from the input time string.
2. If AM/PM is “PM” and hour is not equal to 12, add 12 to the hour value
3. If AM/PM is “AM” and hour is equal to 12, set hour to 0
4. Format the time into 24-hour format and return the result

Python3

import re

def convert_to_24hour(time_str):
    hour, minute, second, am_pm = re.findall('\d+|\w+', time_str)
    hour = int(hour)
    if am_pm == 'PM' and hour != 12:
        hour += 12
    elif am_pm == 'AM' and hour == 12:
        hour = 0
    return f'{hour:02d}:{minute}:{second}'
print(convert_to_24hour('11:21:30 PM'))
print(convert_to_24hour('12:12:20 AM'))

Output

23:21:30
00:12:20

Time complexity: O(1)
Space complexity: O(1)