Per T176033 and T198961 setting the value of "Days to show in watchlist" in `Special:Preferences#mw-prefsection-watchlist` breaks watchlists in that no changes are displayed (in my testing even changes less than 1 minute old were not shown). This value is therefore nonsensical. At T176033#4115252, @Krinkle noted that there are over 1000 users with this setting on Commons alone, suggesting that it previously gave a useful result (this change presumably occurring circa early September 2017).
It is most probable that users a setting a value of zero intended this to mean that that there should be no limit to the number of days displayed, and this would seem to me to be the most logical behaviour going forwards. If no limit is not technically possible, then interpreting 0 as equalling the value of the maximum number of days currently defined for that wiki (i.e. not a hardcoded value) would achieve equally useful results in practical terms.
Similarly the same logic should apply to the "Maximum number of changes to show in watchlist" setting, with 0 meaning "no limit" or "maximum", with the watchlist displaying all changes until whichever of the time or number limits is reached first.
If 0 is equal to "no limit" then MediaWiki should not allow both preferences to be set to this value. If 0 is equal to maximum there will be no issues (that I am aware of) with both being 0.