Update date completions for newer BSDs

-d has been removed in FreeBSD 13 & monterey
-t has also been removed from date(1)
-n has been "Obsolete flag, accepted and ignored for compatibility",
   for a while, leave it out.
-R added for RFC 2822
-I added for ISO 8601

Some description changes

share/completions/date.fish

@@ -2,9 +2,9 @@ complete -c date -f -d "display or set date and time"
 if date --version >/dev/null 2>/dev/null
     complete -c date -s d -l date -d "Display date described by string" -x
     complete -c date -s f -l file -d "Display date for each line in file" -r
-    complete -c date -s I -l iso-8601 -d "Output in ISO-8601 format" -x -a "date hours minutes seconds"
+    complete -c date -s I -l iso-8601 -d "Use ISO 8601 output format" -x -a "date hours minutes seconds"
     complete -c date -s s -l set -d "Set time" -x
-    complete -c date -s R -l rfc-2822 -d "Output RFC-2822 date string"
+    complete -c date -s R -l rfc-2822 -d "Output in RFC 2822 format"
     complete -c date -s r -l reference -d "Display last modification time of file" -r
     complete -c date -s u -l utc -d "Print/set UTC time" -f
     complete -c date -l universal -d "Print/set UTC time" -f
@@ -12,19 +12,27 @@ if date --version >/dev/null 2>/dev/null
     complete -c date -s v -l version -d "Display version and exit" -f
 else
     complete -c date -s u -d 'Display or set UTC time' -f
-    complete -c date -s j -d "Don't actually set the clock" -f
-    complete -c date -s d -d "Set system's value for DST" -x
+    complete -c date -s j -d "Don't try to set the date" -f
 
     set -l uname (uname -s)
 
-    test "$uname" != OpenBSD
-    and complete -c date -s n -d 'Set clock for local machine only' -f
-
     switch $uname
         case FreeBSD Darwin DragonFly
             # only -u is actually POSIX. Rest are BSD extensions:
-            complete -c date -s r -d "Show file mtime, or format seconds" -r
-            complete -c date -s v -d 'Adjust clock +/- by time specified' -f
+            complete -c date -s r -d "Format path mtime or UNIX timestamp" -r
+            complete -c date -s v -d 'Adjust clock ± time specified' -f
+            complete -c date -s R -d "Use RFC 2822 output format"
+
+            set -l release_major (printf '%d' (uname -r) 2>/dev/null)
+
+            # -d,t gone in Monterey, FreeBSD 13
+            if test \( "$uname" = FreeBSD -a "$release_major" -lt 13 \) -o \( "$uname" = Darwin -a "$release_major" -lt 21 \)
+                complete -c date -s t -d "Set system's minutes west of GMT" -x
+                complete -c date -s d -d "Set system's value for DST" -x
+            else
+                complete -c date -s I -d "Use ISO 8601 output format" -x -a "date hours minutes seconds"
+            end
+
         case NetBSD OpenBSD
             complete -c date -s a -d "Change clock slowly with adjtime" -x
             complete -c date -s r -d "Show date given seconds since epoch" -x
@@ -35,6 +43,5 @@ else
             complete -c date -s z -d "Specify timezone for output" -x
     end
 
-    complete -c date -s t -d "Set system's minutes west of GMT" -x
     complete -c date -s f -d 'Use format string to parse date' -f
 end