Time

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.

Moon phase and date
Just as in real life, the NetHack world goes through several phases of the moon. Probably unlike real life, these phases have a real 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. ("You are lucky! Full moon tonight.")
 * Werecreatures, especially at night, will usually be in their animal form.
 * Dogs will howl at night when #chatted to.
 * You have a chance of not being able to tame dogs at night.

During a new moon:
 * You get the message "Be careful! New moon tonight."
 * Contrary to popular belief, Luck is not decreased.
 * 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.

NetHack knows when it is Friday the 13th, and decreases your Luck by one accordingly. 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 -1 and +1 to base Luck will cancel out.

Time
Night is 22:00 - 05:59 according to your local clock. 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.
 * you have a chance of not being able to tame dogs when the moon is full (but you will still make them peaceful and you may try again).
 * gremlins can only steal intrinsics at night.
 * werecreatures can summon more easily.
 * your dog won't whine, yip, bark, or growl, but will always howl when you chat to it, when it is full moon.
 * werecreatures are more likely to be in their were-form, particularly when it is full moon.

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


 * undead do twice as much damage.
 * you get a different message when entering a graveyard ("Run away! Run away!" instead of "You have an uncanny feeling...").

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.