A 30 Day Tourist India Visa holder MUST enter India before the “Date of expiry of ETA”.
Suppose the Date of Expiry of ETA mentioned in you Indian Visa is the 8th of January 2020. A 30-day Visa allows you to stay in India for 30 consecutive days.
If you enter India on the 1st of January 2020, then you can stay until the 30th of January, however if you enter India on the 5th of January, then you can stay in India until the 4th of February.
In other words, the last date of stay in India depends on your entry date into India and is not fixed or known at the time of issue of your India Visa.
It is mentioned in Red bold letters in your Indian Visa: