It's often useful to quickly estimate run-times of programs and algorithms without using a calculator.

One useful mnemonic to remember is:

seconds is a nanocentury

According to this page the kudos for the quote goes to Tom Duff. However, I found it in many sources online that aren't even programming related, so I would take this attribution with a grain of salt.

Anyhow, this mnemonic is surprisingly accurate. Disregarding leap years, there are seconds in a century (100 years). A nanocentury (), therefore, contains 3.1536 seconds, which is less than half a percent away from the real value of .