Steps to replicate the issue (include links if applicable):
What happens?:
Comments before 2016 are not detected by DiscussionTools so no reply button appears. This is because the date format for wuu was changed in 2016 T125047
What should have happened instead?:
They should be parsed as well
Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):
Other information (browser name/version, screenshots, etc.):
I imagine it would be necessary to add an item to the language definition for a list of previously used default date formats, so DiscussionTools can create parsers for all of them.
There are other cases where this happens, e.g. if the timezone or wiki language changed, or if the translation of month names or timezone abbreviations changed. But this situation seemed rare enough, and supporting all these cases seemed tedious enough, that we just didn't do it. I think there's an old bug report about it somewhere.