I need help with automatic display of days

[Hansenjunior]

I have a ETA chart (Estimated time of arrival). At the top of the chart is the time and date( marked in green) Which is filled out when we depart.

My question: How would i go about making the chart update the days automatically based on the the date and time of the departure, and what happens when the clock passes midnight?

The day is determined by the ETA column