Operations between Terms with different units need a daycount to convert
them: one year has 252 days with business/252 and 360 days with
actual/360.
with_daycount() evaluates expr with the given daycount, so
arithmetic and comparison between Terms with different units work.
The Terms are converted to the smallest unit among them (years to months,
months to days).
Outside with_daycount() these operations raise an error.
with_daycount(x, expr)the value of expr.