Alexander Smith Paterson, A Concise System of Theology: Being the Shorter Catechism of the Westminster Assembly of Divines, Analyzed and Explained, p. 221:
Obs. 220. -- The Fourth Commandment requireth us to sanctify one whole day in seven, which God hath expressly appointed to be a holy Sabbath to himself.
By one whole day, as the stated time of worshipping God, we are to understand the same that we are to understand by any other whole day -- namely, a period consisting of twenty-four hours, or what is commonly called a natural day. And this day we should begin and end at the same time that we begin and end any other day, -- namely, at midnight.