The real-world time, date and moon phase affect gameplay. All are determined by your system clock. Technically it is possible to change the clock to get a desired moon phase or time, but this is considered cheating.

On 14 September 2020, the moon is waning crescent.

Just as in real life, the NetHack world goes through several phases of the moon. [1] These have an effect on gameplay.

The date is only checked when the game is started or restored; if you keep playing without saving, you'll continue to see the effects even after the relevant period has passed (or not see the effects even though it has been reached). NetHack divides the month into eight phases of three or four days each; thus the full and new moon will be reported for a rather longer time than your calendar might depict.

During a full moon:

Your base Luck is increased by one. [2] ( "You are lucky! Full moon tonight." )

( ) Werecreatures, especially at night, will usually be in their animal form. [3]

Dogs will howl at night when #chatted to. [4]

You have a chance of not being able to tame dogs at night; failed attempts will still pacify them, and you can try repeatedly.[5]

During a new moon:

You get the message "Be careful! New moon tonight."

Contrary to popular belief, Luck is not decreased. [6]

If a cockatrice uses its hissing attack and you have no lizard corpse, you will always start turning into stone, rather than having a 1-in-10 chance.[7]

NetHack knows when it is Friday the 13th, and decreases your base Luck by one accordingly.[8] The game gives a warning message: "Watch out! Bad things can happen on Friday the 13th."

If Friday the 13th happens to be a full moon, the modifiers to base Luck will cancel each other out. Between 1995 and 2025, Friday the 13th has occurred or will occur during (NetHack's definition of) a new or full moon on the following days:

Date Year Phase September 13 1996 new February 13 1998 full March 13 1998 full August 13 1999 new October 13 2000 full June 13 2003 full March 13 2009 full

Date Year Phase June 13 2014 full November 13 2015 new January 13 2017 full July 13 2018 new September 13 2019 full October 13 2023 new June 13 2025 full

The NAO phase of moon page shows both the current phase of the moon and when it will next be full moon.

Time

Night is 22:00 - 05:59 (10:00pm - 5:59am) according to your local clock, or according to the server's clock if playing remotely. At night:

Your chance of changing into a werecreature when you are a lycanthrope is one in every 60 moves instead of once in every 80 moves. [9]

Gremlins can steal intrinsics. [10]

Werecreatures can summon more easily, and are more likely to be in their were-form, particularly when it is a full moon.

Midnight is 0:00 - 0:59. At midnight:

Undead do twice as much damage. [11]

You get a different message when entering a graveyard ("Run away! Run away!" instead of "You have an uncanny feeling...").[12]

Additionally, vampires can give different messages when #chatted to at night and midnight.

Strategy

While changing the system clock is most certainly cheating, you may prefer to stop playing at night when around hostile gremlins, or at midnight when clearing out a graveyard.

References

This page is based on a spoiler by Dylan O'Donnell. The original license is: Redistribution, copying, and editing of these spoilers, with or without modification, are permitted provided that the following conditions are met: The original contributors to any spoiler must continue to be credited. Any modifications to the spoiler must be acknowledged and credited.