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

Author: Chemaclass

src/assert_dates.sh

@@ -12,10 +12,25 @@ function bashunit::date::to_epoch() {
     ;;
   esac
 
-  # Normalize ISO 8601: replace T with space, strip Z suffix, strip tz offset
+  # Handle Z (UTC) suffix explicitly: BusyBox needs TZ=UTC, BSD needs +0000
+  case "$input" in
+  *Z)
+    local utc_input="${input%Z}"
+    local utc_norm="${utc_input/T/ }"
+    local epoch
+    # GNU/BusyBox: parse in explicit UTC
+    epoch=$(TZ=UTC date -d "$utc_input" +%s 2>/dev/null) && { echo "$epoch"; return 0; }
+    epoch=$(TZ=UTC date -d "$utc_norm" +%s 2>/dev/null) && { echo "$epoch"; return 0; }
+    # BSD: use +0000 offset which %z understands
+    epoch=$(date -j -f "%Y-%m-%dT%H:%M:%S%z" "${utc_input}+0000" +%s 2>/dev/null) && { echo "$epoch"; return 0; }
+    echo "$input"
+    return 1
+    ;;
+  esac
+
+  # Normalize ISO 8601: replace T with space, strip tz offset
   local normalized="$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])