Need a cell to display the modified date for external files

Good Day,

I have workbook, fairly straight forward, it contains a single sheet with a long list of web pages (column A for this example) and some supporting information about the pages. My network hosts the sites, so I have local access to the servers, but they are listed in the sheet as a URL (not the local address). What I need is to know how to get a cell (column B for this example) to display the modified date/timestamp for the file/page referenced in column A. To over-explain: I need to know how to make cell B display the last modified date for the page/file listed in cell A, for about 400 rows. It does not matter if it pulls this data automatically or manually, but I do need it to be able to refresh (not just once and done). I am using Excel 2007.

Thanks very much for your time.
-Joe