diff --git a/internal/assert/assert.go b/internal/assert/assert.go
index a5eed78..ee4115d 100644
--- a/internal/assert/assert.go
+++ b/internal/assert/assert.go
@@ -2,17 +2,29 @@ package assert
 
 import (
 	"reflect"
+	"strings"
 	"testing"
 )
 
+// Equal verifies equality of two objects.
 func Equal[T any](t *testing.T, a T, b T) {
 	if !reflect.DeepEqual(a, b) {
 		t.Fatalf("%v != %v", a, b)
 	}
 }
 
+// NotEqual verifies objects are not equal.
 func NotEqual[T any](t *testing.T, a T, b T) {
 	if reflect.DeepEqual(a, b) {
 		t.Fatalf("%v == %v", a, b)
 	}
 }
+
+// ErrorContains checks whether the given error contains the specified string.
+func ErrorContains(t *testing.T, err error, str string) {
+	if err == nil {
+		t.Fatalf("Error is nil")
+	} else if !strings.Contains(err.Error(), str) {
+		t.Fatalf("Error doen't contain string: %s", str)
+	}
+}
diff --git a/quartz/cron.go b/quartz/cron.go
index 2c1b336..12145fa 100644
--- a/quartz/cron.go
+++ b/quartz/cron.go
@@ -45,7 +45,7 @@ func NewCronTrigger(expression string) (*CronTrigger, error) {
 // NewCronTriggerWithLoc returns a new CronTrigger with the given time.Location.
 func NewCronTriggerWithLoc(expression string, location *time.Location) (*CronTrigger, error) {
 	if location == nil {
-		return nil, errors.New("location is nil")
+		return nil, illegalArgumentError("location is nil")
 	}
 
 	fields, err := validateCronExpression(expression)
@@ -62,7 +62,7 @@ func NewCronTriggerWithLoc(expression string, location *time.Location) (*CronTri
 
 	// full wildcard expression
 	if lastDefined == -1 {
-		fields[0].values, _ = fillRange(0, 59)
+		fields[0].values, _ = fillRangeValues(0, 59)
 	}
 
 	return &CronTrigger{
@@ -158,13 +158,13 @@ func validateCronExpression(expression string) ([]*cronField, error) {
 	}
 	length := len(tokens)
 	if length < 6 || length > 7 {
-		return nil, cronError("invalid expression length")
+		return nil, cronParseError("invalid expression length")
 	}
 	if length == 6 {
 		tokens = append(tokens, "*")
 	}
 	if (tokens[3] != "?" && tokens[3] != "*") && (tokens[5] != "?" && tokens[5] != "*") {
-		return nil, cronError("day field was set twice")
+		return nil, cronParseError("day field set twice")
 	}
 
 	return buildCronField(tokens)
@@ -223,13 +223,13 @@ func parseField(field string, min, max int, translate ...[]string) (*cronField,
 		return &cronField{[]int{}}, nil
 	}
 
-	// single value
+	// simple value
 	i, err := strconv.Atoi(field)
 	if err == nil {
 		if inScope(i, min, max) {
 			return &cronField{[]int{i}}, nil
 		}
-		return nil, cronError("single min/max validation error")
+		return nil, cronParseError("simple field min/max validation")
 	}
 
 	// list values
@@ -247,18 +247,18 @@ func parseField(field string, min, max int, translate ...[]string) (*cronField,
 		return parseStepField(field, min, max, dict)
 	}
 
-	// literal single value
+	// simple literal value
 	if dict != nil {
 		i := intVal(dict, field)
 		if i >= 0 {
 			if inScope(i, min, max) {
 				return &cronField{[]int{i}}, nil
 			}
-			return nil, cronError("cron literal min/max validation error")
+			return nil, cronParseError("simple literal min/max validation")
 		}
 	}
 
-	return nil, cronError("cron parse error")
+	return nil, cronParseError("parse error")
 }
 
 func parseListField(field string, min, max int, translate []string) (*cronField, error) {
@@ -286,17 +286,16 @@ func parseListField(field string, min, max int, translate []string) (*cronField,
 func parseRangeField(field string, min, max int, translate []string) (*cronField, error) {
 	t := strings.Split(field, "-")
 	if len(t) != 2 {
-		return nil, cronError("parse cron range error")
+		return nil, cronParseError(fmt.Sprintf("invalid range field %s", field))
 	}
 
 	from := normalize(t[0], translate)
 	to := normalize(t[1], translate)
 	if !inScope(from, min, max) || !inScope(to, min, max) {
-		return nil, cronError(fmt.Sprintf("cron range min/max validation error %d-%d",
-			from, to))
+		return nil, cronParseError(fmt.Sprintf("range field min/max validation %d-%d", from, to))
 	}
 
-	rangeValues, err := fillRange(from, to)
+	rangeValues, err := fillRangeValues(from, to)
 	if err != nil {
 		return nil, err
 	}
@@ -307,7 +306,7 @@ func parseRangeField(field string, min, max int, translate []string) (*cronField
 func parseStepField(field string, min, max int, translate []string) (*cronField, error) {
 	t := strings.Split(field, "/")
 	if len(t) != 2 {
-		return nil, cronError("parse cron step error")
+		return nil, cronParseError(fmt.Sprintf("invalid step field %s", field))
 	}
 
 	if t[0] == "*" {
@@ -317,10 +316,10 @@ func parseStepField(field string, min, max int, translate []string) (*cronField,
 	from := normalize(t[0], translate)
 	step := atoi(t[1])
 	if !inScope(from, min, max) {
-		return nil, cronError("cron step min/max validation error")
+		return nil, cronParseError("step field min/max validation")
 	}
 
-	stepValues, err := fillStep(from, step, max)
+	stepValues, err := fillStepValues(from, step, max)
 	if err != nil {
 		return nil, err
 	}
diff --git a/quartz/cron_test.go b/quartz/cron_test.go
index f8a05cf..7bbe517 100644
--- a/quartz/cron_test.go
+++ b/quartz/cron_test.go
@@ -355,12 +355,12 @@ func TestCronHourly(t *testing.T) {
 
 func TestCronExpressionInvalidLength(t *testing.T) {
 	_, err := quartz.NewCronTrigger("0 0 0 * *")
-	assert.Equal(t, err.Error(), "invalid cron expression: invalid expression length")
+	assert.ErrorContains(t, err, "invalid expression length")
 }
 
 func TestCronTriggerNilLocationError(t *testing.T) {
 	_, err := quartz.NewCronTriggerWithLoc("@daily", nil)
-	assert.Equal(t, err.Error(), "location is nil")
+	assert.ErrorContains(t, err, "location is nil")
 }
 
 func TestCronExpressionDescription(t *testing.T) {
diff --git a/quartz/error.go b/quartz/error.go
new file mode 100644
index 0000000..14c2c28
--- /dev/null
+++ b/quartz/error.go
@@ -0,0 +1,31 @@
+package quartz
+
+import (
+	"errors"
+	"fmt"
+)
+
+// Errors
+var (
+	ErrIllegalArgument = errors.New("illegal argument")
+	ErrCronParse       = errors.New("parse cron expression")
+	ErrJobNotFound     = errors.New("job not found")
+)
+
+// illegalArgumentError returns an illegal argument error with a custom
+// error message, which unwraps to ErrIllegalArgument.
+func illegalArgumentError(message string) error {
+	return fmt.Errorf("%w: %s", ErrIllegalArgument, message)
+}
+
+// cronParseError returns a cron parse error with a custom error message,
+// which unwraps to ErrCronParse.
+func cronParseError(message string) error {
+	return fmt.Errorf("%w: %s", ErrCronParse, message)
+}
+
+// jobNotFoundError returns a job not found error with a custom error message,
+// which unwraps to ErrJobNotFound.
+func jobNotFoundError(message string) error {
+	return fmt.Errorf("%w: %s", ErrJobNotFound, message)
+}
diff --git a/quartz/error_test.go b/quartz/error_test.go
new file mode 100644
index 0000000..bad168d
--- /dev/null
+++ b/quartz/error_test.go
@@ -0,0 +1,36 @@
+package quartz
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+
+	"github.com/reugn/go-quartz/internal/assert"
+)
+
+func TestIllegalArgumentError(t *testing.T) {
+	message := "argument is nil"
+	err := illegalArgumentError(message)
+	if !errors.Is(err, ErrIllegalArgument) {
+		t.Fatal("error must match ErrIllegalArgument")
+	}
+	assert.Equal(t, err.Error(), fmt.Sprintf("%s: %s", ErrIllegalArgument, message))
+}
+
+func TestCronParseError(t *testing.T) {
+	message := "invalid field"
+	err := cronParseError(message)
+	if !errors.Is(err, ErrCronParse) {
+		t.Fatal("error must match ErrCronParse")
+	}
+	assert.Equal(t, err.Error(), fmt.Sprintf("%s: %s", ErrCronParse, message))
+}
+
+func TestJobNotFoundError(t *testing.T) {
+	message := "for key"
+	err := jobNotFoundError(message)
+	if !errors.Is(err, ErrJobNotFound) {
+		t.Fatal("error must match ErrJobNotFound")
+	}
+	assert.Equal(t, err.Error(), fmt.Sprintf("%s: %s", ErrJobNotFound, message))
+}
diff --git a/quartz/queue.go b/quartz/queue.go
index 1e81ec5..df2e50b 100644
--- a/quartz/queue.go
+++ b/quartz/queue.go
@@ -174,7 +174,7 @@ func (jq *jobQueue) Remove(jobKey *JobKey) (ScheduledJob, error) {
 			return heap.Remove(&jq.delegate, i).(ScheduledJob), nil
 		}
 	}
-	return nil, errors.New("no job with the given key found")
+	return nil, jobNotFoundError(fmt.Sprintf("for key %s", jobKey))
 }
 
 // ScheduledJobs returns the slice of all scheduled jobs in the queue.
diff --git a/quartz/scheduler.go b/quartz/scheduler.go
index 062f2ab..9386c66 100644
--- a/quartz/scheduler.go
+++ b/quartz/scheduler.go
@@ -2,7 +2,7 @@ package quartz
 
 import (
 	"context"
-	"errors"
+	"fmt"
 	"sync"
 	"time"
 
@@ -136,16 +136,16 @@ func (sched *StdScheduler) ScheduleJob(
 	trigger Trigger,
 ) error {
 	if jobDetail == nil {
-		return errors.New("jobDetail is nil")
+		return illegalArgumentError("jobDetail is nil")
 	}
 	if jobDetail.jobKey == nil {
-		return errors.New("jobDetail.jobKey is nil")
+		return illegalArgumentError("jobDetail.jobKey is nil")
 	}
 	if jobDetail.jobKey.name == "" {
-		return errors.New("empty key name is not allowed")
+		return illegalArgumentError("empty key name is not allowed")
 	}
 	if trigger == nil {
-		return errors.New("trigger is nil")
+		return illegalArgumentError("trigger is nil")
 	}
 	nextRunTime, err := trigger.NextFireTime(NowNano())
 	if err != nil {
@@ -224,7 +224,7 @@ func (sched *StdScheduler) GetJobKeys() []*JobKey {
 // GetScheduledJob returns the ScheduledJob with the specified key.
 func (sched *StdScheduler) GetScheduledJob(jobKey *JobKey) (ScheduledJob, error) {
 	if jobKey == nil {
-		return nil, errors.New("jobKey is nil")
+		return nil, illegalArgumentError("jobKey is nil")
 	}
 	scheduledJobs := sched.queue.ScheduledJobs()
 	for _, scheduled := range scheduledJobs {
@@ -232,13 +232,13 @@ func (sched *StdScheduler) GetScheduledJob(jobKey *JobKey) (ScheduledJob, error)
 			return scheduled, nil
 		}
 	}
-	return nil, errors.New("no job with the given key found")
+	return nil, jobNotFoundError(fmt.Sprintf("for key %s", jobKey))
 }
 
 // DeleteJob removes the Job with the specified key if present.
 func (sched *StdScheduler) DeleteJob(jobKey *JobKey) error {
 	if jobKey == nil {
-		return errors.New("jobKey is nil")
+		return illegalArgumentError("jobKey is nil")
 	}
 	_, err := sched.queue.Remove(jobKey)
 	if err == nil {
diff --git a/quartz/scheduler_test.go b/quartz/scheduler_test.go
index c2298ce..fc112a9 100644
--- a/quartz/scheduler_test.go
+++ b/quartz/scheduler_test.go
@@ -68,7 +68,7 @@ func TestScheduler(t *testing.T) {
 
 	nonExistentJobKey := quartz.NewJobKey("NA")
 	_, err = sched.GetScheduledJob(nonExistentJobKey)
-	assert.NotEqual(t, err, nil)
+	assert.ErrorContains(t, err, quartz.ErrJobNotFound.Error())
 
 	err = sched.DeleteJob(nonExistentJobKey)
 	assert.NotEqual(t, err, nil)
@@ -381,21 +381,21 @@ func TestSchedulerArgumentValidationErrors(t *testing.T) {
 	assert.Equal(t, err, nil)
 
 	err = sched.ScheduleJob(nil, trigger)
-	assert.Equal(t, err.Error(), "jobDetail is nil")
+	assert.ErrorContains(t, err, "jobDetail is nil")
 	err = sched.ScheduleJob(quartz.NewJobDetail(job, nil), trigger)
-	assert.Equal(t, err.Error(), "jobDetail.jobKey is nil")
+	assert.ErrorContains(t, err, "jobDetail.jobKey is nil")
 	err = sched.ScheduleJob(quartz.NewJobDetail(job, quartz.NewJobKey("")), trigger)
-	assert.Equal(t, err.Error(), "empty key name is not allowed")
+	assert.ErrorContains(t, err, "empty key name is not allowed")
 	err = sched.ScheduleJob(quartz.NewJobDetail(job, quartz.NewJobKeyWithGroup("job", "")), nil)
-	assert.Equal(t, err.Error(), "trigger is nil")
+	assert.ErrorContains(t, err, "trigger is nil")
 	err = sched.ScheduleJob(quartz.NewJobDetail(job, quartz.NewJobKey("job")), expiredTrigger)
-	assert.Equal(t, err.Error(), "next trigger time is in the past")
+	assert.ErrorContains(t, err, "next trigger time is in the past")
 
 	err = sched.DeleteJob(nil)
-	assert.Equal(t, err.Error(), "jobKey is nil")
+	assert.ErrorContains(t, err, "jobKey is nil")
 
 	_, err = sched.GetScheduledJob(nil)
-	assert.Equal(t, err.Error(), "jobKey is nil")
+	assert.ErrorContains(t, err, "jobKey is nil")
 }
 
 func TestSchedulerStartStop(t *testing.T) {
diff --git a/quartz/util.go b/quartz/util.go
index 0d3fd7c..192913d 100644
--- a/quartz/util.go
+++ b/quartz/util.go
@@ -15,7 +15,7 @@ func indexes(search []string, target []string) ([]int, error) {
 	for _, a := range search {
 		index := intVal(target, a)
 		if index == -1 {
-			return nil, fmt.Errorf("invalid cron field: %s", a)
+			return nil, cronParseError(fmt.Sprintf("invalid cron field %s", a))
 		}
 		searchIndexes = append(searchIndexes, index)
 	}
@@ -50,34 +50,32 @@ func sliceAtoi(sa []string) ([]int, error) {
 	return si, nil
 }
 
-func fillRange(from, to int) ([]int, error) {
+func fillRangeValues(from, to int) ([]int, error) {
 	if to < from {
-		return nil, cronError("fillRange")
+		return nil, cronParseError("fill range values")
 	}
-
 	length := (to - from) + 1
-	arr := make([]int, length)
+	rangeValues := make([]int, length)
 
 	for i, j := from, 0; i <= to; i, j = i+1, j+1 {
-		arr[j] = i
+		rangeValues[j] = i
 	}
 
-	return arr, nil
+	return rangeValues, nil
 }
 
-func fillStep(from, step, max int) ([]int, error) {
+func fillStepValues(from, step, max int) ([]int, error) {
 	if max < from || step == 0 {
-		return nil, cronError("fillStep")
+		return nil, cronParseError("fill step values")
 	}
-
 	length := ((max - from) / step) + 1
-	arr := make([]int, length)
+	stepValues := make([]int, length)
 
 	for i, j := from, 0; i <= max; i, j = i+step, j+1 {
-		arr[j] = i
+		stepValues[j] = i
 	}
 
-	return arr, nil
+	return stepValues, nil
 }
 
 func normalize(field string, dict []string) int {
@@ -97,10 +95,6 @@ func inScope(i, min, max int) bool {
 	return false
 }
 
-func cronError(cause string) error {
-	return fmt.Errorf("invalid cron expression: %s", cause)
-}
-
 func intVal(target []string, search string) int {
 	uSearch := strings.ToUpper(search)
 	for i, v := range target {
@@ -118,7 +112,7 @@ func atoi(str string) int {
 	return i
 }
 
-// NowNano returns the current UTC Unix time in nanoseconds.
+// NowNano returns the current Unix time in nanoseconds.
 func NowNano() int64 {
-	return time.Now().UTC().UnixNano()
+	return time.Now().UnixNano()
 }