Because yesterday was unsuitable to check for the ambiguity how the second
"1" in that expression is actually parsed. Is it parsed as in:
(+1 month 1) thu
as in "go next month, its 1st, forward to Thursday",
or is it:
+1 month (1 thu)
as in "go next month (same day as today, i.e. 2nd NOW), forward 1 thu".
This would skip the weekdays already passed, such as the upcoming
Saturday on Aug 1st. And indeed:
$ date -d '+1 month 1 sat'
Sat Aug 8 00:00:00 CEST 2020
That is: the expression is, alas, *not* good enough to compute the
"first $WEEKDAY of next month". I am at a loss how to do this with
NetBSD's date(1).