Calculate pro rata based on start/end date

Hi everybody,

I do not get my head around how I could solve the following via Excel and was hoping you could help. I would like to calculate the pro rata value of an amount based on start date and end date for the impacted year.

Example: I get an invoice of 1000 USD which covers the period Oct 2017 to February 2018 (5 months). Excel shall now decompose the 1000 USD and assign the appropriate value to 2017 (3 months = 600 USD) and 2018 (2 months = 400 USD).

Note: The date range and values are flexible.

Any ideas how I could do that? Thanks in advance.