As long as contacts are not stored on the SIM card, a birthday can be added to the contacts on the LG phone. To add a birthday, select "Appointments" from the drop-down list. If you wish, the birthday can also be displayed in the calendar.

Android 9 | LG UX 9.0
Step 1:  Open the  Contacts -App
Step 2:  Tap on desired  Contact
Step 3:  Tap on  Edit
Step 4:  Tap on  More
Step 5:  Tap on  Events
Step 6:  Tap on  Type
Step 7:  Choose  Birthday
Step 8:  Tap on  Date
Step 9:  Choose  Date  and tap on  OK
Step 10:  Tap on  Save
  1. Open the Contacts-App
  2. Tap on desired Contact
  3. Tap on Edit
  4. Tap on More
  5. Tap on Events
  6. Tap on Type
  7. Choose Birthday
  8. Tap on Date
  9. Choose Date and tap on OK
  10. Tap on Save

LG Instructions