Yes, any part of a day was counted as a day, UNLESS is is more particularly specified. Three days AND three nights means . . . . three days AND three nights.
Lol, there is no UNLESS in the Scriptures.
Why do you think the disciples noted that Sunday was the THIRD DAY since Jesus died??
The Scriptures absolutely lock in Sunday as the day Jesus rose. Once that day is locked in, it is very simple to count backwards to find out the day He died.
3, 2, 1
( Wednesday or Thursday are absolutely eliminated as possibilities )