diff --git a/cron.go b/cron.go
index 60f21e0..7db53cc 100644
--- a/cron.go
+++ b/cron.go
@@ -2,7 +2,7 @@ package gocron
 
 import "time"
 
-type timeUnit interface {
+type TimeUnit interface {
 	// Next returns the next iteration of a schedule and `true` when valid,
 	// otherwise it returns a time after `next` and `false`.
 	Next(next time.Time) (time.Time, bool)
@@ -10,7 +10,7 @@ type timeUnit interface {
 
 // Schedule is a representation of a Cron expression.
 type Schedule struct {
-	timeUnits []timeUnit
+	timeUnits []TimeUnit
 }
 
 // Parse returns a schedule from the Cron expression and returns an error if the
diff --git a/cron_test.go b/cron_test.go
index 5b403da..1b707af 100644
--- a/cron_test.go
+++ b/cron_test.go
@@ -46,6 +46,7 @@ func TestSchedule_Next(t *testing.T) {
 		{"* * * * * 5L", time.Date(2000, time.March, 31, 0, 0, 0, 0, time.UTC)},
 		{"* * * * * 4L", time.Date(2000, time.March, 30, 0, 0, 0, 0, time.UTC)},
 		{"* * * 31 * L", time.Date(2001, time.March, 31, 0, 0, 0, 0, time.UTC)},
+		{"* * * ? * 0L", time.Date(2000, time.March, 26, 0, 0, 0, 0, time.UTC)},
 	}
 
 	for _, v := range vectors {
diff --git a/error.go b/error.go
new file mode 100644
index 0000000..6921209
--- /dev/null
+++ b/error.go
@@ -0,0 +1,45 @@
+package gocron
+
+import (
+	"errors"
+	"fmt"
+)
+
+var (
+	ErrMultipleNotSpecified = errors.New("only one `?` is supported")
+)
+
+type TimeUnitError struct {
+	inner error
+	kind  TimeUnitKind
+}
+
+func newTimeUnitErr(kind TimeUnitKind, inner error) TimeUnitError {
+	return TimeUnitError{inner: inner, kind: kind}
+}
+
+func (e TimeUnitError) Error() string {
+	return fmt.Sprintf("time unit `%s` malformed: %s", e.kind, e.inner)
+}
+
+func (e TimeUnitError) Is(err error) bool {
+	tue, ok := err.(TimeUnitError)
+	return ok && tue.kind == e.kind && errors.Is(tue.inner, e.inner)
+}
+
+type TimeUnitKind int
+
+const (
+	Seconds TimeUnitKind = iota
+	Minutes
+	Hours
+	Days
+	Months
+	WeekDays
+)
+
+var kinds = []string{"seconds", "minutes", "hours", "days", "months", "week days"}
+
+func (k TimeUnitKind) String() string {
+	return kinds[k]
+}
diff --git a/parser.go b/parser.go
index aea3ccb..1d47f01 100644
--- a/parser.go
+++ b/parser.go
@@ -29,30 +29,34 @@ func (p Parser) Parse(expression string) (schedule Schedule, err error) {
 
 	weekdays, err := p.parse(matches[5], convertWeekDay, 0, 6)
 	if err != nil {
-		return schedule, err
+		return schedule, newTimeUnitErr(WeekDays, err)
 	}
 	months, err := p.parse(matches[4], convertUnit, 1, 12)
 	if err != nil {
-		return schedule, err
+		return schedule, newTimeUnitErr(Months, err)
 	}
 	days, err := p.parse(matches[3], convertWithLastDayOfMonth, 1, 31)
 	if err != nil {
-		return schedule, err
+		return schedule, newTimeUnitErr(Days, err)
 	}
 	hours, err := p.parse(matches[2], convertUnit, 0, 23)
 	if err != nil {
-		return schedule, err
+		return schedule, newTimeUnitErr(Hours, err)
 	}
 	minutes, err := p.parse(matches[1], convertUnit, 0, 59)
 	if err != nil {
-		return schedule, err
+		return schedule, newTimeUnitErr(Minutes, err)
 	}
 	seconds, err := p.parse(matches[0], convertUnit, 0, 59)
 	if err != nil {
-		return schedule, err
+		return schedule, newTimeUnitErr(Seconds, err)
 	}
 
-	schedule.timeUnits = []timeUnit{
+	if isNotSpecified(weekdays) && isNotSpecified(days) {
+		err = ErrMultipleNotSpecified
+	}
+
+	schedule.timeUnits = []TimeUnit{
 		monthTimeUnit{units: months},
 		dayTimeUnit{units: days},
 		weekdayTimeUnit{units: weekdays},
@@ -68,6 +72,10 @@ func (Parser) parse(expr string, convFn ConvertFn, min, max int) (fields []ExprF
 	if expr == "*" {
 		return
 	}
+	if expr == "?" {
+		fields = append(fields, notSpecifiedExpr{})
+		return
+	}
 
 	for _, u := range strings.Split(expr, ",") {
 		var field ExprField
@@ -134,6 +142,28 @@ func parseInterval(expr string, convFn ConvertFn, min, max int) (i intervalExpr,
 	return
 }
 
+// isNotSpecified returns true if any of the field is not specified.
+func isNotSpecified(fields []ExprField) bool {
+	for _, field := range fields {
+		if _, ok := field.(notSpecifiedExpr); ok {
+			return true
+		}
+	}
+	return false
+}
+
+// notSpecifiedExpr is an expression field that represent a question mark `?`,
+// or in other words a value not specified.
+type notSpecifiedExpr struct{}
+
+func (notSpecifiedExpr) Compare(_ time.Time, other int) Ordering {
+	return OrderingEqual
+}
+
+func (notSpecifiedExpr) Value(_ time.Time, other int) int {
+	return other
+}
+
 // unitExpr is an expression field that represents a single possible value.
 type unitExpr int
 
diff --git a/parser_test.go b/parser_test.go
new file mode 100644
index 0000000..ea39d37
--- /dev/null
+++ b/parser_test.go
@@ -0,0 +1,82 @@
+package gocron
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestParser_Parse_refusesTwiceNotSpecified(t *testing.T) {
+	_, err := Parser{}.Parse("* * * ? * ?")
+	requireErrorIs(t, err, ErrMultipleNotSpecified)
+}
+
+func TestParser_Parse_abortsOnMalformedSeconds(t *testing.T) {
+	_, err := Parser{}.Parse("a * * * * *")
+
+	var e TimeUnitError
+	requireErrorAs(t, err, &e)
+	requireSameKind(t, e.kind, Seconds)
+}
+
+func TestParser_Parse_abortsOnMalformedMinutes(t *testing.T) {
+	_, err := Parser{}.Parse("* a * * * *")
+
+	var e TimeUnitError
+	requireErrorAs(t, err, &e)
+	requireSameKind(t, e.kind, Minutes)
+}
+
+func TestParser_Parse_abortsOnMalformedHours(t *testing.T) {
+	_, err := Parser{}.Parse("* * a * * *")
+
+	var e TimeUnitError
+	requireErrorAs(t, err, &e)
+	requireSameKind(t, e.kind, Hours)
+}
+
+func TestParser_Parse_abortsOnMalformedDays(t *testing.T) {
+	_, err := Parser{}.Parse("* * * a * *")
+
+	var e TimeUnitError
+	requireErrorAs(t, err, &e)
+	requireSameKind(t, e.kind, Days)
+}
+
+func TestParser_Parse_abortsOnMalformedMonths(t *testing.T) {
+	_, err := Parser{}.Parse("* * * * a *")
+
+	var e TimeUnitError
+	requireErrorAs(t, err, &e)
+	requireSameKind(t, e.kind, Months)
+}
+
+func TestParser_Parse_abortsOnMalformedWeekDays(t *testing.T) {
+	_, err := Parser{}.Parse("* * * * * a")
+
+	var e TimeUnitError
+	requireErrorAs(t, err, &e)
+	requireSameKind(t, e.kind, WeekDays)
+}
+
+// --- Utilities
+
+func requireErrorIs(t testing.TB, err, target error) {
+	t.Helper()
+	if !errors.Is(err, target) {
+		t.Fatalf("expected error: %#v, found %#v", target, err)
+	}
+}
+
+func requireErrorAs(t testing.TB, err error, target any) {
+	t.Helper()
+	if !errors.As(err, target) {
+		t.Fatalf("expected error of kind: %T, found %#v", target, err)
+	}
+}
+
+func requireSameKind(t testing.TB, a, b TimeUnitKind) {
+	t.Helper()
+	if a != b {
+		t.Fatalf("%s != %s", a, b)
+	}
+}
diff --git a/time-unit.go b/time-unit.go
index a2577de..4442d93 100644
--- a/time-unit.go
+++ b/time-unit.go
@@ -8,6 +8,7 @@ type secTimeUnit struct {
 	units []ExprField
 }
 
+// Next implements TimeUnit.
 func (s secTimeUnit) Next(next time.Time) (time.Time, bool) {
 	if len(s.units) == 0 {
 		// Expression is `*`.
@@ -32,6 +33,7 @@ type minTimeUnit struct {
 	fields []ExprField
 }
 
+// Next implements TimeUnit.
 func (m minTimeUnit) Next(next time.Time) (time.Time, bool) {
 	if len(m.fields) == 0 {
 		// Expression is `*`.
@@ -56,6 +58,7 @@ type hourTimeUnit struct {
 	fields []ExprField
 }
 
+// Next implements TimeUnit.
 func (h hourTimeUnit) Next(next time.Time) (time.Time, bool) {
 	if len(h.fields) == 0 {
 		// Expression is `*`.
@@ -80,6 +83,7 @@ type dayTimeUnit struct {
 	units []ExprField
 }
 
+// Next implements TimeUnit.
 func (d dayTimeUnit) Next(next time.Time) (time.Time, bool) {
 	if len(d.units) == 0 {
 		// Expression is `*`.
@@ -110,6 +114,7 @@ type monthTimeUnit struct {
 	units []ExprField
 }
 
+// Next implements TimeUnit.
 func (m monthTimeUnit) Next(next time.Time) (time.Time, bool) {
 	if len(m.units) == 0 {
 		// Expression is `*`.
@@ -134,6 +139,7 @@ type weekdayTimeUnit struct {
 	units []ExprField
 }
 
+// Next implements TimeUnit.
 func (wd weekdayTimeUnit) Next(next time.Time) (time.Time, bool) {
 	if len(wd.units) == 0 {
 		return next, true