Hours which are past midnight are suffixed with next day date. In Panchang day starts and ends with sunrise.
Sankranti is a phenomenon when the Sun transits from one zodiac to another zodiac while travelling through zodiac belt. There are twelve Sankranti in a solar year. Sankranti is also known and spelt as Sankraman or Sankranthi or Sankramanam.
When the Sun (Surya Dev) enters the Sagittarius sun sign or the dhanu rashi, Dhanu Sankranti is celebrated.