Date and time calculations and automatic date changes

Dear Readers,

I would greatly appreciate any assistance you can provide.

I must prepare a spreadsheet at work calculating salary amounts for the fiscal year (July 1, 2007 - June 30, 2008) that include salary amounts from two annual merit increase dates as follows:

Employee: Jane Doe

Annual Merit Increase Dates Fiscal Year Dates
4/1/2007 - 3/31/2008 $100,000 7/1/2007 - 3/31/2008 9 mos $75,000
4/1/2008 - 3/31/2009 $105,000 4/1/2008 - 6/30/2008 3 mos $26,250
Total FY salary $101,250

Please let me know how excel can automatically calculate the dates to show the number of months or pay periods (24 semi-monthly) and how much of each yearly merit increase amount to be paid in the fiscal year period?

I would also like to know if there is a formula for automatically updating the annual dates without changing them manually?

Thanks so much for your help!