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. Now, Let’s see How to Convert AM/PM to 24 Hour Time using Python.

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 the 12-hour clock and 12:00:00 on the 24-hour clock.

Examples:

Input : 11:21:30 PM
Output : 23:21:30
Input : 12:12:20 AM
Output : 00:12:20

Convert AM/PM to 24 Hour Time using Slicing

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

# 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")) 

20:05:45

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

Convert time from 12 to 24 hour format Using DateTime

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.

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'))  
print(convert24('12:12:20 AM'))

23:21:30
00:12:20

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

Convert time from 12 to 24 hour Using RegEx

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.

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

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'))  

23:21:30
00:12:20

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