Declension table of manvantara

Neuter	Singular	Dual	Plural
Nominative	manvantaram	manvantare	manvantarāṇi
Vocative	manvantara	manvantare	manvantarāṇi
Accusative	manvantaram	manvantare	manvantarāṇi
Instrumental	manvantareṇa	manvantarābhyām	manvantaraiḥ
Dative	manvantarāya	manvantarābhyām	manvantarebhyaḥ
Ablative	manvantarāt	manvantarābhyām	manvantarebhyaḥ
Genitive	manvantarasya	manvantarayoḥ	manvantarāṇām
Locative	manvantare	manvantarayoḥ	manvantareṣu

Compound manvantara -

Adverb -manvantaram -manvantarāt