fix(assert): normalize Z suffix to +0000 for date parsing

Author: Chemaclass

src/assert_dates.sh

@@ -12,10 +12,16 @@ function bashunit::date::to_epoch() {
     ;;
   esac
 
-  # Normalize ISO 8601: replace T with space, strip Z suffix, strip tz offset
+  # Normalize Z suffix to +0000 (understood by both GNU and BSD date)
+  case "$input" in
+  *Z)
+    input="${input%Z}+0000"
+    ;;
+  esac
+
+  # Normalize ISO 8601: replace T with space, strip tz offset
   local normalized="$input"
   normalized="${normalized/T/ }"
-  normalized="${normalized%Z}"
   # Strip timezone offset (+HHMM or -HHMM) at end for initial parsing
   case "$normalized" in
   *[+-][0-9][0-9][0-9][0-9])