Another reading relates time's generosity to its being favorable toward the patron as a consequence of his generosity, while locating its miserliness in a different respect or period.
The crucial qualification is in the same respect. Calling one subject generous and miserly at the same time, about the same matter and in the same sense, would be problematic. Different respects or periods allow the two descriptions to coexist.
The interpretation must nevertheless respect the poem's sequence and verbal forms rather than invoke a distinction that the wording cannot bear.