The first solution is to pass the required data as a function argument. Suppose that the payment function from the Mortgage sub-module can take a days_per_year keyword argument, then the Calculator module can just pass the value as follows:

# Calculator.jl
module Calculator

const days_per_year = 365

include("Mortgage.jl")
using .Mortgage: payment

function do_something()
    return payment(1000.00, 3.25, 5; days_per_year = days_per_year)
end

end # module

Hence, the Mortgage sub-module does not really need to reference the days_per_year symbol from Calculator anymore, reducing any unnecessary dependency.