I’ve had to work with authors to fix time line issues numerous times. If you describe a character doing something that takes place at midnight, and then you have them doing something that might take three hours at the most, you can’t have dawn breaking when they finish. Especially if it’s in an unbroken scene where there’s no indication of a time lapse. I once spent hours trying to untangle an author’s time line, because it started out with one, and then split into two sets of characters on different time lines. At first you didn’t realize they were on different ones because when specific times were mentioned in one line, it wasn’t mentioned in the other one. There were a few things that seemed a little off in each one, but they were easily corrected and weren’t that noticeable until you started breaking down the time line. The problem really surfaced when two characters, one from each line, came together and were pretty much together for the rest of the book. Even though it wasn’t the only big issue, one of the major problems was that dawn had broken for one character, but when she came together with the other character, they were running around in the dark and dawn broke a second time. Either I had somehow missed an entire day during the reading, or the author didn’t keep track of their time line. We discovered the issue was the latter.
I usually see this in books with more than one POV. This is especially true if the author likes to mention specific times. And yes, a book can follow this character or certain events on one line for a few chapters, and then the book starts following another character through the same time line or sequence of events. That’s not a problem, and it’s done frequently. The problem comes when you follow one character through a long stretch of time or sequence of events and then start following another character through that same stretch of time without indicating you’ve gone back to the beginning. If you do go through a long period of time and then switch to another character, it’s helpful to indicate this at the beginning of the chapter. Perhaps something like:
There are other formats an author might choose, but the above is one of the most common examples. And by “long stretch of time”, I mean if you start with your first character, pretty much begin with the start of their day, and then follow them until suppertime or even bedtime. If you don’t use some kind of time indicator at the beginning of the chapter, and you just start with your second character having breakfast, people are going to assume it’s the next day. So when you get to the part that evening where your two characters are on the phone discussing the events of the day, and your readers realize they’re talking about the same day, there’s bound to be a moment of confusion.
However, I’ve also seen this happen in the same time line, which appeared to be pretty linear. I’ve seen characters arrive at work at 9:00 a.m., do some things that might only keep them occupied until around 1:00 p.m. or 2:00 p.m. at the latest, including accounting for travel and the time spent at each location, and suddenly it’s 7:00 p.m. Normally, this isn’t a problem when there’s an indicator that there’s been a time lapse and the characters did some stuff “off-screen”. But this was presented in a linear fashion where we were with the characters every step of the way. Nope. That doesn’t work.
Unraveling a convoluted time line takes a lot of time and work. In some cases, the author can fix it easily with a scene break to indicate the time lapse, giving a chapter a time indicator, eliminating specific time references, etc. Other times it can cause entire scenes to be rewritten, moved, or eliminated. So for the sanity of your editor and yourself (Yes, fixing a time line can cause Fried Brain Syndrome. Consider yourself warned.), please try to keep track of your time line. You can use a style sheet or create a file to help keep you straight on which events occur when, and where each character is during those events, during specific times, or on specific days. Just use a format that makes the most sense to you, whether it's a spreadsheet, writing it out on notebook paper, using note cards, using a calendar...the available options and formats are endless. The only thing that matters is that it works for you. In some stories, it’s pretty easy to keep track of. In others…well, it’s easy to create a mess if you aren’t careful.