Fluid time control in a visual novel is an interesting concept. VNs–even the most interactive ones–are meant to follow set scripts. That is why VNs typically restrict the player’s ability to manage time: offering that much freedom to the user could mean exponentially more paths to write. Morning, afternoon, evening settings offer a convenient compromise. Those time blocks enable time-of-day events, while also controlling the passage of time so plotlines are easier to compose.
If I was to incorporate fluid time control into my VN, it would likely be through prompting the player for the amount of time they want to stay at a location, rather than a skip time feature (I’m sure spamming the skip button is not fun). Building the foundation of fluid time through prompts in theory sounds rather simple. There would be a calendar variable (set?) and a time variable. Each time entry increases the time variable until we reach 24:00 (midnight), at which time we increment the calendar day by one and reset the time to 0:00.
For example, let’s say the user opts to go to the bar. A certain amount of time passes to get to the bar, then the interface asks, “How long do you plan to stay at the bar?”. A short stint would mean they only have time for a quick drink. A longer allotment would open up the possibility of the character doing other stuff at the bar (e.g. striking up conversations, playing darts, watching the big game, getting into bar fights). If the time the player enters goes beyond the closing time of the bar, the VN triggers an end scene whereby you get kicked out of the bar.
The problem with open time management is that you have to set constraints to it. The character shouldn’t be able to stay awake for days on end. At some point, the player will get hungry, or bored. Perhaps they have to go to work and make money. If your VN had a set opening structure for each day–say, waking up in your bed and doing some morning routine–you need to make sure there is some continuity so the transitions make sense (i.e. the character shouldn’t end up everyday passing out at midnight and somehow waking up in their bed).
Now, bear in mind that I am thinking of VNs that run on a day-by-day, slice-of-life model (think Soft Fantasies or, to some extent, Worshippers of the Gain). These games for the most part do not require you to remember many events and when they happened. If you do need to keep some event in mind, the VN generally tracks it for you with a variable/stat. This allows the VN to remind you that said event has continuing influence (e.g. “You have 3 days left to do [task].”). It sounds like the timing of events has more significance in your VN, so you may have to create a major events log for the player to be able to refer back to.
I am curious to learn what the advantages are of giving the player fluid time control? Aside from permitting the user to go at their own pace, are you letting them skip scenes they find boring? It might help to also know what you are using to create your VN. Hope this helps!