Can anyone here explain to me how the Hindu calendar works?

So I checked my birthday from Hindu calendar on this website called janmtithi.in (https://janmatithi.in/index.html) They gave me the tithi of phalguna amavasya (15th) and in Gregorian Calendar that's 29th March but in kalnirnay ( a brand of calendar) that day is showing chaitra amavasya so it's not confusing