I have a column of times which span some twenty-four-hour-long interval (not necessarily starting at midnight or noon). The first one is filled in manually and then each subsequent one is autofilled according to the formula "`[prevcell]+TIME(0,30,0)`

", where the previous cell is the cell immediately above the current one. The first cell is always going to be on the hour mark, so it will be of form "n:00:00" for some integer n. Thus, all subsequent times will either be on the new hour mark or on the thirty-minute mark, meaning that they will be of form "n:m0:00" where n is an integer and m is either 0 xor 3.

Then I tried to use conditional format the cells so that any cell which is on a thirty-minute mark (id est: is of the form "n:30:00" for integer n) has a light grey background color (whereas all other cells are white/no-color). So, for example, a cell with 10:00:00 in it will be white, but a cell with 10:30:00 would be light grey.

I did this:

Rule type: "Format only cells which contain".

Format cells with: "cell value", "equal to", "

`=TIME(HOUR([curcell]), 30, 0)`

". (Where the current cell is the particular cell being formatted itself.)Selected appropriate background color formatting.

The formatting worked for most cells. I would perhaps understand if it did not work for the first or last cells for some reason, but it does usually. For what it does not work are exactly the cells with the following values: 11:30:00, 18:30:00 (6:30:00 p.m.), 21:30:00 (9:30:00 p.m.), 22:30:00 (10:30:00 p.m.); these cells are not colored (white) when they should be colored. It works for all other cells, including 23:30:00 (11:30:00 p.m.), even though it did not work for the cell for 11:30:00 (a.m.). There are no cells which are colored when they should not be; the only cells which are not colored when they should be are the aforementioned ones.

When I change the seed value/time in my topmost cell, different errors of the exact same sort arise (often but not always for the same values and sometimes for additional ones).

Does anyone know what is wrong or how to fix this?

**Edit!** Here are some observations which I have had:

- It does not work for any time after the next midnight, no matter what the time in my first cell is (except 00:00:00). The dependence on the first cell's value seems to go like this:
- 11:30 (a.m.) works for certain times in my first cell.
- It does not work for 00:00:00, 00:30:00, 1:00:00, 1:30:00, and 2:00:00.
- It works for 2:30, 3:00, 3:30.
- It does not work for 4:00, 4:30, 5:00, 5:30, 6:00, 6:30.
- It works for all times after 7:00, incrementing by 30 minutes, until and including 11:30. (Consideration no longer applies after that seed value).

- I did not check extensively, but from a quick glance: 18:30:00, 21:30:00, and 22:30:00 never work.
- Depending on the value of my first cell, certain other n:30:00 times do not work (for integer n), but I cannot discover a pattern. There are no errors, as far as I can likewise tell, for any n:00:00 times (for integer n).

**Update #1**

I have changed the formatting condition to: Format cells with "cell value", "not equal to", "`=TIME(HOUR([curcell]), 0, 0)`

". (Where the current cell is the particular cell being formatted itself.)

This seems to have improved the situation some, but not perfectly. It formats everything correctly except for the fact that it improperly colors cells with these time: 20:30:00 (8:00:00 p.m.), 23:00:00 (11:30 p.m.), and (if present) all times after and including the next midnight (24:00:00; a.k.a.: the next 12:00 a.m.).

(**Edit**: The times for which it improperly colors seem to depend on the value of my first cell and perhaps other factors. But there are always two discrete errors later in the day and the after-midnight problem.)

**Update #2**

The main problem has been solved. However, if anyone can tell me why it arose in the first place, I would be very thankful. Curious minds want to know!