variable: fix outdated time shift for variable with TypeTime (#39720)

close pingcap/tidb#39719

sessionctx/variable/variable.go

@@ -383,6 +383,10 @@ func (sv *SysVar) checkTimeSystemVar(value string, vars *SessionVars) (string, e
 	if err != nil {
 		return "", err
 	}
+	// Add a modern date to it, as the timezone shift can differ across the history
+	// For example, the Asia/Shanghai refers to +08:05 before 1900
+	now := time.Now()
+	t = time.Date(now.Year(), now.Month(), now.Day(), t.Hour(), t.Minute(), t.Second(), t.Nanosecond(), t.Location())
 	return t.Format(FullDayTimeFormat), nil
 }
 

sessionctx/variable/variable_test.go

@@ -669,3 +669,21 @@ func TestSkipSysvarCache(t *testing.T) {
 	require.True(t, GetSysVar(TiDBGCScanLockMode).SkipSysvarCache())
 	require.False(t, GetSysVar(TiDBEnableAsyncCommit).SkipSysvarCache())
 }
+
+func TestTimeValidationWithTimezone(t *testing.T) {
+	sv := SysVar{Scope: ScopeSession, Name: "mynewsysvar", Value: "23:59 +0000", Type: TypeTime}
+	vars := NewSessionVars(nil)
+
+	// In timezone UTC
+	vars.TimeZone = time.UTC
+	val, err := sv.Validate(vars, "23:59", ScopeSession)
+	require.NoError(t, err)
+	require.Equal(t, "23:59 +0000", val)
+
+	// In timezone Asia/Shanghai
+	vars.TimeZone, err = time.LoadLocation("Asia/Shanghai")
+	require.NoError(t, err)
+	val, err = sv.Validate(vars, "23:59", ScopeSession)
+	require.NoError(t, err)
+	require.Equal(t, "23:59 +0800", val)
+}