The command +today (see +HELP +TODAY) tells you what the current IC time is. The compression is 1:1, which means that one IC day passes for every RL day.
You are not required to obey time slavishly. For example, f you log on and find that it is night, you can roleplay it as morning or evening. If you enter a public scene, however, it is always wise to ask whether time is being played according to +today or if something else has been agreed upon.