<span class="searchmatch">decenniī</span> genitive singular of decennium...
<span class="searchmatch">decenniīs</span> dative/ablative plural of decennium...
(“year”) + -is (suffix forming compound adjectives). decennium n (genitive <span class="searchmatch">decenniī</span> or decennī); second declension decennium; decade Second-declension noun...