it's = it is

All other times use "its."

Treat "its" as a substitute for a possesive pronoun. For example:

The diesel threw a rod yesterday. Some say its time had come. Just do the "It is" test. If you cannot substitute "it is" for 'its" then "its" is correct.

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You don't need a weatherman to know which way the wind blows - Robert A. Zimmerman