Convert dates to Serial

Hello,

The Question:
How can I convert a date stored as string into a serial format?


Why I need it:
I found that to update the values of a pivot according to a date using VBA the date have to be in serial format.

I use this code to do that:
Workbooks(Name).Sheets("Zara").Range("G28").Value = PT.GetPivotData("Sum of Average", "Company", 1, "YearMonth", pivot_date).Value


I got the date as a string in a cell and I tried to conver it into serial using the Format function with 'c' but it didn't work

pivot_date=Format(str_date, c)

There is other way to convert String Into Serial(date)?

Thanks,
Pedro F. Pardo