Python: Elapsed time without using system clock

I need to measure the elapsed time of a program that waits for an input.

However, my system clock will not be stable when measuring this - I am using a Pi 0 that does not have an RTC, so on bootup the clock is off. However, the clock will sync to NTP when it connects to the internet. Waiting for internet before starting the program is not an option.

I am currently using `time.time()` at the beginning and end, and subtracting the values.

start = time.time()
# wait for input and complete tasks
end = time.time()
elapsed=end-start

This fails because when the NTP syncs, it can change the value of time.time() by over 20 minutes. This drastically changes my elapsed time.

I would like a similar system of timing, however even when the system clock changes, it stays constant.

Solution

You can use the time.monotonic() function in Python to measure elapsed time without relying on the system clock. time.monotonic() returns a monotonically increasing time in seconds since an arbitrary point in the past, and it is not subject to changes due to NTP synchronization or other system time adjustments. Here's how you can use it:

import time

start = time.monotonic()
# Wait for input and complete tasks
end = time.monotonic()
elapsed = end - start

print(f"Elapsed time: {elapsed} seconds")