Short answer: There is no one fixed clock-time for ekasana.

What to follow:

• Take the single meal only in daylight. Start after Navkārshī (about 48 minutes after sunrise) and finish before sunset.
• Do not eat or drink after sunset.
• When you take pratyākhyān (the vow), specify your chosen time window (e.g., Navkārshī, Porsi, Sādh‑porsi) and keep it consistently that day.
• Eat in one sitting, at one place, without restarting the meal later.

Tradition notes:

• Shvetāmbara practice commonly names the daytime slots: Navkārshī (≈48 min after sunrise), Porsi (≈3 hours after sunrise), Sādh‑porsi (≈4½ hours). Any one of these is acceptable for ekasana if stated in the vow.
• Digambara practice also requires the single meal to be taken in daylight and completed before sunset; the exact hour is left to one’s vow and local ācār.

Essence: Ekasana is about one meal in one sitting during daylight, taken within the time you declare in your vow, and never at night.