diff --git a/internal/controller/utils.go b/internal/controller/utils.go
index 3092c379..072ace1f 100644
--- a/internal/controller/utils.go
+++ b/internal/controller/utils.go
@@ -7,6 +7,7 @@ import (
 	"log"
 	"maps"
 	"net"
+	"regexp"
 	"slices"
 	"strconv"
 	"strings"
@@ -576,13 +577,25 @@ func getPeerCertName(etcdClusterName string) string {
 	return peerCertName
 }
 
+// validityDurationDayPrefix matches a leading integer day segment, e.g. "365d" or the "100d" in "100d12h".
+var validityDurationDayPrefix = regexp.MustCompile(`^(\d+)d`)
+
 // parseValidityDuration parses a duration string and returns the parsed duration.
 // If the customizedDuration is empty, it returns the defaultDuration.
+// A leading day segment (e.g. "365d", "100d12h") is expanded to hours, since
+// time.ParseDuration does not support the "d" unit.
 // Returns an error if the duration string cannot be parsed.
 func parseValidityDuration(customizedDuration string, defaultDuration time.Duration) (time.Duration, error) {
 	if customizedDuration == "" {
 		return defaultDuration, nil
 	}
+	if m := validityDurationDayPrefix.FindStringSubmatch(customizedDuration); m != nil {
+		days, err := strconv.Atoi(m[1])
+		if err != nil {
+			return 0, fmt.Errorf("failed to parse ValidityDuration: %w", err)
+		}
+		customizedDuration = fmt.Sprintf("%dh%s", days*24, customizedDuration[len(m[0]):])
+	}
 	duration, err := time.ParseDuration(customizedDuration)
 	if err != nil {
 		return 0, fmt.Errorf("failed to parse ValidityDuration: %w", err)
diff --git a/internal/controller/utils_test.go b/internal/controller/utils_test.go
index 67592316..6260b7e9 100644
--- a/internal/controller/utils_test.go
+++ b/internal/controller/utils_test.go
@@ -1090,3 +1090,33 @@ func TestCreateCMCertificateConfig(t *testing.T) {
 		})
 	}
 }
+
+func TestParseValidityDuration(t *testing.T) {
+	tests := []struct {
+		name        string
+		input       string
+		expected    time.Duration
+		expectedErr bool
+	}{
+		{name: "day suffix", input: "365d", expected: 365 * 24 * time.Hour},
+		{name: "days and hours", input: "100d12h", expected: 100*24*time.Hour + 12*time.Hour},
+		{name: "plain go duration", input: "24h", expected: 24 * time.Hour},
+		{name: "minutes", input: "90m", expected: 90 * time.Minute},
+		{name: "empty returns default", input: "", expected: certInterface.DefaultCertManagerValidity},
+		{name: "invalid string", input: "abc", expectedErr: true},
+		{name: "day unit without value", input: "d", expectedErr: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			duration, err := parseValidityDuration(tt.input, certInterface.DefaultCertManagerValidity)
+			if tt.expectedErr {
+				require.Error(t, err)
+				assert.ErrorContains(t, err, "failed to parse ValidityDuration")
+				return
+			}
+			require.NoError(t, err)
+			assert.Equal(t, tt.expected, duration)
+		})
+	}
+}