ionic start ionic-local-notification blank --v2
cd ionic-local-notification
ionic platform add android
ionic plugin add cordova-plugin-background-mode
npm install --save @ionic-native/background-mode
ionic plugin add de.appplant.cordova.plugin.local-notification
npm install --save @ionic-native/local-notifications

import { BackgroundMode } from '@ionic-native/background-mode';
import { LocalNotifications } from '@ionic-native/local-notifications';

constructor(
    ..., 
    private bgMode: BackgroundMode,
    private localNoti: LocalNotifications
) 

constructor(
    ...
){
    platform.ready().then(() => {
      ...
      this.bgMode.enable();
      this.bgMode.on('activate').subscribe(() => {
          var today = new Date();
          var pm_6 = new Date();
          pm_6.setDate(today.getDate());
          pm_6.setHours(18);
          pm_6.setMinutes(0);
          pm_6.setSeconds(0);
          var at_6_pm = new Date(pm_6);
          this.localNoti.schedule({
            text: 'แจ้งเตือนตอน 6 โมง',
            firstAt: at_6_pm,
            every: 'day'
          });
      })
    });
}

this.bgMode.enable();

this.bgMode.on('activate').subscribe(() => {
...
})

var today = new Date();
var pm_6 = new Date();
pm_6.setDate(today.getDate());
pm_6.setHours(18);
pm_6.setMinutes(00);
pm_6.setSeconds(0);
var at_6_pm = new Date(pm_6);

this.localNoti.schedule({
    text: 'แจ้งเตือนตอน 6 โมง',
    firstAt: at_6_pm,
    every: 'day'
});