Choviar / Chovihar time in Jainism means the period after sunset until sunrise the next day, during which Jains who observe chovihar do not eat or drink anything, including water. ( jainknowledge.com)

So, the exact chovihar time changes daily and by location, because it depends on your local sunset and sunrise. ( jainknowledge.com)

In short: after sunset = chovihar starts; after sunrise = chovihar ends. ( jainknowledge.com)