Create Table that logs the value and time each time a singe cell is updated.

[Thargyll]

Hi,

I stumbled upon a problem I cannot solve.

I've created a portfolio that is updated with web queries. I now have a single cell that refreshes every minute and shows the current net portfolio worth.
Would it be possible to create a table from that cell that records each 30 minutes the current value of the portfolio and assigns a timestamp to it so I can use this data to create a historical graph of the portfolio.

Any help would be much appreciated,

Thargyll