Is there a way to add timer for 12 hours interval?

i think you can only adjust the interval through time by repeating every hour:

• Interval - Move the slider to the desired amount of time before the timer repeats. For example, 15 minute intervals will activate at 2:00, 2:15, 2:30, etc.
• Chat Lines - This is the minimum amount of chat lines (measured in lines per 5 minutes) required to activate the timer. This is useful in slow chats to prevent Nightbot from spamming in an empty channel.

You could have two timers that your turn on and off yourself if you want them at different intervals during different times of the day.

https://docs.nightbot.tv/control-panel/timers

Hey @Hearty Sloth!

You can adapt this command to suit your needs, instead of using the $(uptime) variable, use the $(time) variable.