parser: return error instead of panic when TZ prefix has no fields

parser.go

@@ -95,6 +95,9 @@ func (p Parser) Parse(spec string) (Schedule, error) {
 	if strings.HasPrefix(spec, "TZ=") || strings.HasPrefix(spec, "CRON_TZ=") {
 		var err error
 		i := strings.Index(spec, " ")
+		if i < 0 {
+			return nil, fmt.Errorf("timezone prefix missing schedule fields: %v", spec)
+		}
 		eq := strings.Index(spec, "=")
 		if loc, err = time.LoadLocation(spec[eq+1 : i]); err != nil {
 			return nil, fmt.Errorf("provided bad location %s: %v", spec[eq+1:i], err)
@@ -313,6 +316,9 @@ func getRange(expr string, r bounds) (uint64, error) {
 	if step == 0 {
 		return 0, fmt.Errorf("step of range should be a positive number: %s", expr)
 	}
+	if step > r.max-r.min {
+		return 0, fmt.Errorf("step (%d) above maximum (%d): %s", step, r.max-r.min, expr)
+	}
 
 	return getBits(start, end, step) | extra, nil
 }

parser_test.go

@@ -334,6 +334,18 @@ func TestStandardSpecSchedule(t *testing.T) {
 			expr: "* * * *",
 			err:  "expected exactly 5 fields",
 		},
+		{
+			// Step value larger than the field range should be rejected.
+			// */90 in the minute field (range 0-59) would only ever fire at
+			// minute 0, silently ignoring the intent.  See issue #543.
+			expr: "*/90 * * * *",
+			err:  "step (90) above maximum (59)",
+		},
+		{
+			// Same check for hours: */30 is the max valid step (0-23 → range 23).
+			expr: "* */30 * * *",
+			err:  "step (30) above maximum (23)",
+		},
 	}
 
 	for _, c := range entries {
@@ -362,6 +374,18 @@ func every5min(loc *time.Location) *SpecSchedule {
 	return &SpecSchedule{1 << 0, 1 << 5, all(hours), all(dom), all(months), all(dow), loc}
 }
 
+// TestParseTZMissingFields verifies that a TZ/CRON_TZ prefix with no
+// trailing schedule fields returns an error rather than panicking with a
+// slice-bounds-out-of-range crash. See GitHub issue #554.
+func TestParseTZMissingFields(t *testing.T) {
+	cases := []string{"TZ=0", "TZ=UTC", "CRON_TZ=America/New_York"}
+	for _, spec := range cases {
+		if _, err := ParseStandard(spec); err == nil {
+			t.Errorf("ParseStandard(%q): expected error for missing schedule fields, got nil", spec)
+		}
+	}
+}
+
 func every5min5s(loc *time.Location) *SpecSchedule {
 	return &SpecSchedule{1 << 5, 1 << 5, all(hours), all(dom), all(months), all(dow), loc}
 }