Month
A month was usually the amount of time it took for a moon to orbit its planet. This was usually a portion of a year, and a large number of days (in which case it may be broken down into weeks).

On Earth, a month was originally the length of the lunar cycle (29.53 days). Most calendars at some point made the month one twelfth of a solar year (30.44 days). In Earth's most common calendar, the months were either 30 or 31 days long, with one shorter month (February) having 28 or 29 days depending on the year.

In August of 2151, Matthew Ryan told Travis Mayweather he ate his last steak eighteen months prior. ()

On an uninhabited planet it only experienced night four days a month due to its binary stars. ()

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Time measurements