How to Convert HH:MM:SS Strings to Seconds in Python – Multiple Approaches Explained

How to Convert HH:MM:SS Strings to Seconds in Python

This piece is part of a series that shares interesting technical insights gathered from various sources, aiming to spark curiosity and broaden thinking during study breaks.

Problem Statement

Given a time string such as "20:15:31", we need to compute the total number of seconds (20 × 3600 + 15 × 60 + 31 = 72931 seconds).

Using the time Module

The built‑in time module can parse the string into a struct_time object, but the default year is 1900, which is earlier than the Unix epoch.

import time
time.strptime('20:15:31', '%H:%M:%S')[:6]
# (1900, 1, 1, 20, 15, 31)

Converting this struct to a timestamp with time.mktime fails because the year is before 1970. Adding a dummy year (1970) and compensating for the local timezone offset yields the correct result.

import time
t = time.strptime('1970 20:15:31', '%Y %H:%M:%S')
seconds = time.mktime(t) - time.timezone
# 72931.0

Using the datetime Module

The datetime module behaves similarly, returning a datetime with year 1900.

from datetime import datetime
dt = datetime.strptime('20:15:31', '%H:%M:%S')
# datetime.datetime(1900, 1, 1, 20, 15, 31)

Because the year is before the epoch, we can compute the difference from a reference date (e.g., 1900‑01‑01) and read the seconds attribute.

from datetime import datetime
delta = datetime.strptime('20:15:31', '%H:%M:%S') - datetime(1900, 1, 1)
seconds = delta.seconds  # 72931

Using python-dateutil

The third‑party python-dateutil parser attaches the current date to the time, allowing direct subtraction.

from dateutil.parser import parse
seconds = (parse('20:15:31') - parse('0:0:0')).seconds
# 72931

Direct Arithmetic Calculation

By splitting the string on ':' and multiplying by the appropriate factors, we can compute the total seconds without any library.

seconds = sum(map(lambda x, y: x * y, map(int, '20:15:31'.split(':'), [3600, 60, 1]))
# 72931

NumPy can perform the same operation more compactly.

import numpy as np
seconds = np.sum(np.array(['20', '15', '31'], dtype='int32') * [3600, 60, 1])
# 72931

Conclusion

The article demonstrates several ways to turn an "HH:MM:SS" string into seconds in Python, highlighting pitfalls such as year offsets and timezone adjustments, while also showing that a simple arithmetic approach can be the most straightforward solution.