Dropbox app shows invalid date format in XLSX files when saved in Libreoffice

I know this has been asked before so let me start by saying that I am only asking again because I had found a workaround for it in the past, but cannot remember what it was and was not smart enough to bookmark the page. I also want to point out that there is a slight difference between my issue and the one I found on these forums, so that's another reason I bring it up again. Here is the link I found of someone else asking this question:

https://discussions.apple.com/thread/4274543

Here are the specifics to my situation: I have an iPhone 7 currently running iOS 12.3.1 (but this has been a problem for me on older iOS versions, too). It started when I switched from Excel to Libreoffice.

Starting from the beginning, when I save a .XLSX file using Libreoffice from my Windows 10 PC, then I view the file in the dropbox app in preview mode on my iPhone 7, the dates all show incorrectly. For example, 06/22/2019 shows as 06/173/2019. As has been established, 173 is the day of the year for the date given (June 22 is the 173rd day of the year). The same thing happens if I change the format to something similar (i.e. one-digit month and/or two-digit year) or something completely different. Here are the other formats I have tried:

June 173, 2019

173. June 2019

So clearly, the problem is specifically with the day of the month (not the month itself or the year) and seems to be format independent. The problem also occurs only if I save the file in Libreoffice, then view it from the Dropbox app on my iPhone. (In the link above, I think they mentioned the problem using Libreoffice AND OpenOffice, but I have not tested with OpenOffice.)

I do not see the problem if I view it in Libreoffice on my computer (that's probably obvious). I also don't see the problem if I view the in preview mode from the dropbox website on my computer (or if I select Open With, then select an online app like Microsoft Excel online). I also don't see the problem if I view it using the Polaris Office app on my iPhone. To add to that, if I save the file using Excel from a PC, Excel Online from the Dropbox website, or using Polaris Office on my iPhone, it will then display the dates correctly from the iPhone Dropbox app in preview mode. The problem returns, however, if I edit the file later using Libreoffice and then save it.

What's different for me is that I am using XLSX files, not XLS, whereas the person who reported this issue previously was using XLS files, then when they switched to XLSX files, they had a different issue.

I read that it has something to do with the iOS parser for these files (which presumably, the Dropbox app is using), but again, I recall reading somewhere that there is a workaround for when I save the files in Libreoffice that will make the date format show correctly in the iPhone Dropbox app. I just don't remember what it was. Does anybody happen to know it?