In both drama the main leads start communicating with each other through an object but living in different timeline. In Il Mare it's trough a Mailbox, while in Toki no Kaori it's via ham radio.

Toki No Kaoiri is the Japanese remake of Korean movie Ditto. Leads from different timelines communicate through a ham radio.

Special Encounter is what Botox supposed to be. Botox was never released at the end.
But we got Special Encounter which is the Chinese adaptation of the manhwa Botox.

I Will Be Your Ear (2021) version is basically the full-length movie version of this Music Video/short film by HANDSIGN.

Both drama has
- Older Woman/Younger man
- Office setup
- Boss/employee, superior/subordinate relationship

Older woman/Younger man relation with office(superior/subordinate or boss/employee) setup. Takane no Hana-san is light-hearted though.

In both dramas time rift in a certain space connects the main leads together.
In "Until I Meet September's Love" it's the apartment wall of the FL whereas in "Il Mare" it's the postbox.

In both drama one of the lead assist the other to find love, while doing so they eventually develop feelings for each other.

Couple's relationship in disarray, male lead gets a chance to re-evaluate their life and relationship by going back time(Shitteru Waifu) or turning into his younger-self(18 Again)

Couples relationship collapsed/about to collapsed. Male lead/Female lead or both goes back time to relive or change their life but fate brings em together.