This page is designed to tell you how long, in the real world, you need to wait until the time in Gaia Online's ''Gaia Standard Time'' switches to Dawn or Dusk.
To use it, first you must find out what time it is now, in Gaia Standard Time. This url can tell you that: What is Gaia Standard Time?
Get the time in GST from that page, and then add the following to this page's URL:
Do not include the colon. For example, if the time in GST were 12:45, you would add ?gaiatime=1245 to the URL of this page. Don't worry about AM or PM; since dawn starts at 6:00 AM and dusk starts at 6:00 PM, and there's no difference between the two as far as what you'll experience in Towns/Fishing, it doesn't much matter.
Give it a try!