Hi there, the battery manager is showing the time of last full 100% battery charged.
Please follow below steps may improve battery backup:
1. Close apps that are not used (Bluetooth, WiFi, GPS, etc.)
2. Adjust sleep time and brightness (Settings > Display)
3. Boost memory when you see memory consumption is high with no app running,
4. check for background running applications, stop/disable unwanted apps, you can manage apps from auto-start manager.
You may also follow the below steps:
1. Clear system cache.
2. Try the same under safe mode:
Hold the power button for 5 seconds => When the screen flashes with option => Press & hold power option => You will get an option to restart in safe mode
3. If still you face the issue
factory reset your phone after taking a full backup :
https://www.asus.com/support/FAQ/1006346/