There is a history of a bug in which Smart Groups do not store the proper relative date ("Next 60 days") criteria, but rather convert the relative date into a hard date ("3 Jan 2018" to "4 Mar 2018"). This bug was noticed starting with v. 4.7.x and was. supposedly fixed in 4.7.13 I'm still having this issue in 4.7.27 (Joomla 3.8.2).
Anyone else notice this recurring? Any advice?