Here is the concise idea for Śvetāmbara fasting (upavās) timings:

• Start of the fast (upavās): Usually after the previous sunset. This is called Rātrijalātyāg or Ratri Bhojan Tyāg — you refrain from eating after sunset. Some people may have boiled water or light intake, depending on the exact upavās type you follow (e.g., Tivihar, Chauvihar, etc.). Always follow the guidance of your local sangh or guru.
• End of the fast (parṇa): Break the fast the next morning after sunrise, most commonly at Navkārsī time (about 48 minutes after sunrise). This is the typical Shvetāmbara practice for a daily upavās. If your sangh has announced a specific parṇā window, follow that exact time.

Notes:

• During Paryuṣaṇa, many Śvetāmbara follow the strict practice of not eating after sunset (Ratri Bhojan Tyāg) and may limit water as per local custom.
• Digambar practices differ in some details (e.g., exact allowed foods and water timings), so the timings above are specific to Śvetāmbara norms.
• For precise daily timings, check with your local Jain temple or sangh because sunrise/sunset times vary by location and date.

You can read more here and