GNU date recognizes "next x" and "last x", for example:

$ date --date="next month"
Fri Jul  7 11:44:46 AM CEST 2023
$ date --date="last month"
Sun May  7 11:44:52 AM CEST 2023

uutils date, using humantime_to_duration, doesn't recognize such dates and returns an "invalid date" error.