Chronos is a simple mod to control the length of a Minecraft day.
It has no configuration file, and a single command: /timescale.
To set the day length to twice the normal length:
/timescale set 0.5
To set the day length of half the normal length:
/timescale set 2
To view the current time scale for the world you are in:
/timescale
Time scale is stored per dimension in a file called Chronos.dat.