This year (2023) Hebcal shows that April 19 is the 28th of Nisan. Does that mean that the 28th of Nisan starts at sundown on April 19 or that the 28th of Nisan started at sundown on the 18th of April? And why isn't this fundamental issue clearly address

Hi, thanks for using Hebcal.

If you have questions about Hebrew to Gregorian date conversion, we'd encourage you to use our Hebrew Date Converter page. It's very easy to convert in both directions, and we'd encourage you to experiment with the date conversions so you can get comfortable understanding when the Hebrew date changes.

For example,

Tue, 18 April 2023 after sunset = 28th of Nisan, 5783

https://www.hebcal.com/converter?gd=18&gm=4&gy=2023&gs=on&g2h=1

28th of Nisan, 5783 = Wed, 19 April 2023

https://www.hebcal.com/converter?hd=28&hm=Nisan&hy=5783&h2g=1

Wed, 19 April 2023 = 28th of Nisan, 5783

https://www.hebcal.com/converter?gd=12&gm=3&gy=2023&g2h=1

We hope this helps!