mc-nbt-edit 0.4 by sakisds <sakisds@gmail.com>
Uses the NBT library by Thomas Woolford <woolford.thomas@gmail.com>
Based on the NBT specifications by Markus Persson

This script is capable of creating and editing tags inside NBT structures, mainly used by Minecraft.
For usage please use the --help option.

I created this to edit the level.dat files from minecraft saves but it should work with any other nbt files too. Here is a list of level.dat settings it can modify:

Data.Time (long): Stores the current "time of day" in minecraft. Value should be between 0 and 24000. Here are some reference points:
-0 = start of daytime
-12000 = start of sunset
-13800 = start of nighttime
-22200 = start of sunrise
-24000 = daytime again.
Data.hardcore (byte): Hardcore mode off and on. Possible values are 0(false) and 1(true).
Data.MapFeatures (byte): Whether structures (dungeons, mob villages, etc.) will be generated. Possible values are 0(false) and 1(true).
Data.GameType (int): Whether in survival (0) or in creative (1) mode.
Data.rainTime (int): Time until the next weather change. Value can be anything, though to change the weather you probably want to set this to something like 1.
Data.thunderTime (int): Same as above, but for thunders.
Data.LevelName (string): Specifies the name of the level.
Data.Player.health (int): Amount of health the player has. Legal values are between 0 and 20 but it can be set higher.

You can find more values and their descriptions here:
http://www.minecraftwiki.net/wiki/Alpha_Level_Format