On 18 December 2025 (Thursday), according to the standard Indian lunar calculation:

• Till about 2:30–2:35 am (IST) – it is Krishna Paksh Trayodashi (Vad Teras)
• After that for the rest of the day – it is Krishna Paksh Chaturdashi (Vad Chaudas)

Shwetambar Jain panchangs that follow Ramchandra Surishwarji Maharaj’s tradition also use the same basic lunar tithi system, so for all practical Jain observances on that civil date, it will be taken as Vad Chaudas (Margshirsh/Posh Krishna Chaudash), unless your local panchang for your city shows a slight shift.

Because exact tithi start–end times can differ by:

• city (local sunrise), and
• which printed/digital panchang of Ramchandra Suriji’s lineage you follow,

you should still confirm the precise time from your local “Ramchandra Suriji” Jain panchang or temple notice if you need it for pachchakkhan, parna, or special niyam.

For a very simple explanation of what “tithi” is and why it can differ by place or panchang method, you can read more here