daynight_reset - resets the settings to default.
daynight_enabled 1/0 - enable or disable the day and night system.
daynight_paused 1/0 - pause the current cycle where it is.
daynight_realtime 1/0 - enable or disable whether the cycle uses your server's local time as the cycle.
daynight_log 1/0 - toggle console logs (for that slight extra performance I would leave this off, it's more for debug)
daynight_length_day 1-3600 - sets how long the day cycle will last.
daynight_length_night 1-3600 - sets how long the night cycle will last.
daynight_time - gets the current cycle time
daynight_settime 0-24 - sets the current cycle time
This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
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