I knew I'd have to deal with it sometime, and it finally caught up with me today. My Master Cosmere Timeline spreadsheet has far too many relative dates, and not enough absolutes.
Roshar's date system
The biggest reason I have put it off is that the date system Brandon made up is both supremely logical and at the same time totally crazy. A year has five hundred days, but there's also a thousand-day cycle with different highstorms around the new year. In each year there are ten months of fifty days each. The months are broken into ten five-day weeks. The date indicates what year, month, week of the month, and day of the week it is and looks like this: 122.214.171.124. It is impossible for me to do the math in my head to decide what the date would be 37 days ago, so I don't use the dates in my reckoning, and only calculate them as an afterthought. This dating system is also a hassle because two weeks in our world is almost three weeks there, and a month there is almost two of ours, and when writing Brandon doesn't even pretend to pay attention to those differences.
Day numbers in The Way of Kings
But then we have to talk about my relative date system. The timeline of The Way of Kings is a mess. The story for Shallan starts more than 100 days earlier than Dalinar's storyline. And Kaladin is roughly 50 days different from that. So for that book I had to pick a day when I knew there was crossover between the viewpoints and work forward and back from there. So a date in The Way of Kings might be marked on my spreadsheet as D 23 or K-57.
Day numbers in Words of Radiance and Oathbringer
For Words of Radiance I started over at day 1 for that book. Those numbers count up until the new year which is day 71. Oathbringer starts just after the new year, so I used the day of the year for my book-specific day number. Of course switching systems at the start of each book made it hard for me to calculate just how many days there were between events in WOR and OB. So I put in another column which indicated a relative number of days counting before and after the arbitrary date of the end of WOR.
The next problem I dealt with were the line items that say something like "five years ago" for their date. With more than a year of onscreen time from the first chapters of The Way of Kings to the end of Oathbringer, it's really necessary to note that it's five years before what event with a solid date. Once I have a date to assign to it, I also have to decide how exact the date is. When I come back three years from now I will need to know whether this date is firm, or if it would be okay to put it three or four months on either side.
Putting it all together
When Peter found an error in the spreadsheet one day, I decided to match a serial number to each date after the year 1160 (which makes for easy calculating), and make that my absolute day number from here until forever (though I'll probably still make a book relative date, since it's a useful way to talk about things with the rest of the team). To find the Roshar dates from the serial numbers I made another spreadsheet with a vlookup table for the dates and serial numbers, then translated all the dates from the three books into that single new system (finding several more errors as I went).