Merry meet @danielle8!
Good question! If a fellow witch told you they only cast at moonset, then that’s fine for them and their practice, but I’d like to assure you that it doesn’t mean that you have to follow the same rule. Casting only at moonset is a pretty short window for casting - you’ll find that plenty of witches cast at other times, too!
For those who cast moon magick, you’ll find witches charging their moon water or magickal tools all night. Some witches like to cast during the “Witching Hour” (generally believed to be between 3 and 4am). Other witches will cast moon spells whenever they can see the moon in the sky - which may be into the morning or even the early afternoon.
You can even cast spells without the moon at all - from Solar Magick with the Sun or other planetary forces, to magick that involves other, non-celestial forces such as Ocean Magick or Elemental Spellwork.
All this is to say: it’s perfectly fine and normal to cast when the time is right for you and your schedule 
Blessed be!