How convert date in to tithi?

To convert a date into a tithi (lunar day) in the Hindu calendar, you first need to determine the phase of the moon on that specific date. Tithis are based on the lunar cycle, where each tithi corresponds to a specific phase of the moon, starting from the new moon (Amavasya) to the full moon (Purnima) and back. You can use lunar calendars or software that calculates tithis according to the Hindu Panchang. Additionally, local observances may influence the exact tithi based on regional practices.