Calculate Future Dates From Start Date With Varying Time Period/cycle

I need to determine a formula which will allow me to calculate a future date based upon a current date with varying time periods.

For example:

I have a bill which is paid on the 15th and last business day of each month. I would like to be able to see the next due date regardless of what day of the week it is.

I have a bill which is paid every other Tuesday. I would like to know the next due date without having to enter +14 for every due date in the future. In other words, it is preferable to be able to open the spreadsheet and automatically see the next due date, not use autofill to repeatedly add +14 to a previous date which would limit the # of future due dates that could be calculated.

I have a bill which is paid on the last business day of each month, not the last Friday of each month. I would need excel to return a value for the last day of the month which = Monday-Friday, regardless of what day of the week it may be as long as it isn't Saturday or Sunday(holiday exclusion would be nice but not required).