I suggest that you use a spreadsheet software, because it allows you to combine a flexible timeline with a flexible outliner.
This solution has the following advantages:
You can – but you don't have to – use dates.
You can also use days of the week without calendar dates, or any other time scheme you want (e.g. an unspecified chronological order such as "first", "then", "even later"). If you want dates, spreadsheet software supports calculations and ordering on dates.
You can use text of any length and are not limited to chapter titles (as in timelining software) or a brief synopsis (as in an outliner).
Each story element can belong to multiple storylines, not just one as in all common timeliners and outliners.
As most of us have used a spreadsheet software before, there is no learning curve. Open and work.
A more detailed explanation
I'm currently outlining a middle grade novel with three parallel storylines, and today I was struggling a bit with figuring out how to synchronize the different storylines. Does the son solve dad's dating troubles before he solves the burglary case, or does he catch the thief first? Stuff like that.
I began outlining in Scrivener, my writing tool of choice, but I soon got lost on the "corkboard". I'm using labels to signify storylines, and arranging the "index cards" by label on Scrivener's corkboard gives my a nice overview of how the story develops over the different storylines:
But as I was rearranging the index cards, rewriting their synopses, and moving the whole story forward and backward in time, I was often confused what day I was seeing in the current window. I wanted certain events to take place on a Friday afternoon, because a government office the protagonist needed to visit would be closed over the weekend, but when I inserted a day and moved the whole storyline back one day and then moved other events around, I no longer knew what was happening when, because Scrivener doesn't display dates on the corkboard. Maybe it does, when I set up the dates in Aeon Timeline, but when I imported the Scrivener project into Aeon Timeline and had to choose dates and times and realized how long it would take me to set this all up when all I needed was the day of the week and a division by morning, afternoon, and evening, I gave up and instead installed the demo for Patchwork, which I have recommended in my other answer to this question.
It took me about fifteen minutes to download and install Patchwork, but when I began setting up the timeline there (choosing "8:00-12:00" for "morning"), I realized another problem: the timeline in Patchwork only shows the title of a chapter, but not the synopsis (see the screenshot in my other answer).
I realized that what I wanted was a combination of corkboard and timeline on which I would be able to both work on the chapter (or scene) synopses and see the time the event takes place, but with a flexible time system where I don't have to chose hours and minutes, if all I need is the day of the week and the afternoon.
Then it came to me. I remembered how I had developed my characters in Excel and that a corkboard and timeline are both basically tables.
But I also remembered how I had struggled with developing characters in Excel for a previous project, because there is a limit (of 409.5 points or 546 pixels) for how high a row in Excel can be. That's usually not a problem when you use Excel for data, but when you use it to write outlines you will want all the text visible all the time, but in Excel text will often be longer than a row's maximum height and get cut off. You can, of course, merge cells to create higher cells, but this destroys the parallelisation of character traits that the rows in a cell represent, and I don't really want to merge cells every time I hit he max cell height.
And while I sat in front of my computer with the Scrivener corkboard open and thought about this, I noticed that in fact this is a problem in Scrivener's corkboard, too! In Scrivener the index cards all have the same size, and text that is longer than fits on the card is not shown while you don't work on that card. This makes both Excel and the Scrivener corkboard useless for outlining, unless you limit yourself to brief scenes or shorthand synopses.
Fortunately for me my Microsoft Office license has expired and I had been looking for a replacement for Excel because I didn't want to pay 99 Euro a year for an Office 365 license for my family, and when I opened my old Excel character development sheet in Numbers I was pleasantly surprised to find that Apple's Numbers doesn't have a maximum row height (or not one that I have managed to exceed yet), and it is free (if you have a Mac – and I have three!), so I began to outline my story in numbers, and I realized another advantage of a spreadsheet software over a timelining application or a corkboard: the spreadsheet app allows you to have one scene belong to multiple storylines! And this makes perfect sense and is in fact essential, because storylines intersect, and anything from half a sentence to a couple of paragraphs in any scene or chapter can belong to another storyline than the rest of it.
So what I finally did, and will happily use for my next project again, was create a spreadsheet in Numbers with the first three columns representing the timeline – day of the week, date, and time (in my case "morning", "afternoon", "evening", and "night") –, and then three columns (or as many as you like) for the outliner – in my case three storylines, highlighted in different colors: