Calculating elapsed time with time.time() resulted in negative value

I have a python app that runs a loop continuously.
XUBUNTU 20.04.
Python 3.8
To calculate the elapsed time, I use the time module. End of the loop, app sleeps to ensure the loop interval is constant. Here is a sample:

import time

scan_time = 0.1
while True:
    tic = time.time()
    do_work1()
    toc = time.time()
    et = toc - tic
    print(f"Elapsed time for Work1 was {et}")

    do_work2()
    toc = time.time()
    et = toc - tic
    print(f"Overall elapsed time was {et}")
    
    if et < scan_time:
        time.sleep(scan_time - et)

I had complaints from users that the app hangs from time to time. Once I observed that the
print(f"Overall elapsed time was {et}") had printed a large negative number (equal -6124.428). That means the sleep time was a large positive number and indeed the app had fallen to a deep sleep. Why on earth would the et become negative?

Thanks