In Jain fasting, the safe and traditional rule is:

• Drink only boiled, filtered water during daylight.
• Stop at your local sunset; after sunset, no water.

Common details by tradition:

• Shvetambar: Boiled water is taken only in the day, usually starting after Navkarsi (about 48 minutes after sunrise) and ending at sunset.
• Digambar: Many observe nirjal upvaas (no water at all). If water is permitted, it is limited to daylight and stopped at sunset.

Practical tip: Check your local sunset time and finish your last sip a little before sunset to be sure.