Near the end of the Uva Letzion prayer, there is a phrase that refers to ימות המשיח. This sounds like a feminine ending. Shouldn't the phrase be ימי המשיח as the word יום is masculine?
Joüon and Muraoka's A Grammar of Biblical Hebrew calls this a poetic plural (see section 98f). They note that in both cases, ימות is parallel to שנות, which exhibits the same phenomenon: the plural construct of שָׁנָה is either שְׁנֵי or שְׁנוֹת, with twice as many examples of the former as the latter in Tanakh (but still 20 cases of the latter). I think the comment was intended to suggest that ימות takes that form to match שנות. Indeed, they write that the standard plural forms יָמִים and יְמֵי were used (instead of *יוֹמִים and *יוֹמֵי) "probably on the analogy of שָׁנים, שְׁנֵי", and if that's the case, then matching forms of יוֹם by analogy to forms of שָׁנָה already has precedence.
However, they also point out that the non-construct form שנת and ימת appear in the Tel Siran inscription (see line 7), so there is potential for an independent plural form יָמוֹת and שָׁנוֹת that are simply unattested in Tanakh, but reflected in the construct forms we see here.