Decline 'rēs, reī' (f.) — thing, matter — in all cases (5th declension).
Hint: 5th declension has short -eī in genitive/dative singular (note: diēī has long ē before ī).
Decline 'diēs, diēī' (m.) — day — in all cases (5th declension).
Hint: diēs is one of the few masculine 5th declension nouns. Note long ēī (vs rēs which has short eī).