During that day

How it works

A day has a rim. Civil midnight is the usual rim: 00:00 to the next 00:00 in local time. During that day is every hour strictly inside that rim. 23:59 is still during Thursday if Thursday is the name. 00:01 is during Friday. The phrase is a container, not a direction. It does not mean “in those days” and it does not mean “someday.”

During a day, without “that,” is the generic interior: sometime in a twenty-four-hour civil square, or sometime while you are awake on it. It is useful for rules (“take this during the day, not at night”) and useless for appointments (“we’ll talk during a day” is not a time). Add the weekday when the container has to be found: during that Thursday, during Tuesday’s hours.

Workplaces split the interior. Morning, afternoon, and evening are not fixed clocks. A hospital day shift and a hospital night shift can both happen during the same weekday and still never meet. If you needed overlap, you needed clock times, not the word day.

Time zones split the container too. During that day in one city can be during the previous day in another. Date a file with the place, or “during that day” will attach to the wrong weekday for the other person.

Hours inside the named square

To use the phrase as a plan, fill the square: Name the weekday and the date. Name the local zone if two places are in play. Put clock times inside the midnight rim, or say that you mean waking hours instead.

If a shift crosses midnight, split it: the part before 00:00 is during that day; the rest is during the next. During that day at a school is usually the bell-to-bell hours, which are a subset of the civil square. During that day at a shop is the posted hours for that weekday, which may be shorter on Sunday than on Thursday. The phrase borrows the hours of the institution if you are standing in the institution. It borrows midnight if you are standing in a statute or a log.

Do not stretch “during that day” to cover the evening before. Sunday night is during Sunday. It can feel like Monday, and it is still Sunday. Monday starts at Monday’s midnight, or at Monday’s alarm if you are speaking loosely — and if you are speaking loosely, you will fight about whether 23:30 Sunday was “the night before” or “during that day.” Pick the rim once.

During a day versus across days

Across days is a span: during Tuesday and Wednesday, over the week, from Friday into Saturday. During that day forbids the span. If a package moves overnight, part of the journey is during one weekday and part during the next. Tracking that wants two containers, or a clock, not one “during that day” sticker on the whole trip.

Medicine labels that say “during the day” mean the waking half, opposed to night. They do not mean a named weekday. A once-daily dose during that day still needs a clock or a meal, or “during” becomes “whenever.” The weekday only matters if the dose is tied to a dated start: first tablet on Monday, then every civil day, not every waking stretch you remember.

During that day means inside one square. If you crossed midnight, you are already during another weekday, even if the task did not feel finished. A diary line “during that day I…” should carry the weekday in the heading. Without it, later you will not know which square you meant, and the hours will float. The interior is only useful if the container is labeled.