It's not a sundial. It's either a small turbinid or a trochid, and either should be safe in your aquarium. If it's a turbinid, it will have an operculum made of shelly material (it will look like a small pearl in the aperture, with a dent in the middle if it's in the subfamily Colloniinae). If it's a trochid, it will either have a transparent amber disc for an operculum or it will lack one entirely. From the single pic, it looks like it might be a Pseudostomatella or Stomatolina species, which wouldn't have an operculum.
Cheers,
Don