Proto-Celtic *dīyos

→ Breton deiz (inherited)

→ Breton hiziv (mention)

→ Welsh dydd (inherited)

→ Welsh heddiw (mention)

→ Welsh eleni (mention)

→ Irish dia (inherited)

→ Cornish dedh (inherited)

→ Old Welsh did (inherited)

→ Old Irish día (inherited)

→ Middle Welsh dyd (inherited)