I haven't studied the issue of the calendar too much in-depth, but I recently came across one reformed author who believes its biblically meant to be sunrise to sunrise.


"The most plausible passage for an evening-reckoning of the Sabbath in the New
Testament, concerning the events of Jesusí burial and the buying of spices on Friday
evening (Lk. 23:50-56), is found to be incompatible with an evening-reckoning and
consistent with the morning-view. "

I'm not sure it materially affects the sign of Jonah the Saviour was intending.