Data capture method

What I am trying to do is log data from a DDE linked cell. It gets real time updates from a Allen Bradley Data Highway network. I want to capture this data every 15 minutes and log it into the excel sheet for the 24 hour period.
I have used the VBE for the sheet and put this code in worksheet event change

Range("A65535").End(xlUp).Offset(1, 1).Value = Now
Range("A65535").End(xlUp).Offset(1, 0).Value = Range("A1").Value

Only problem it does the deed every time the cursor is moved
How do I creat a 15 minute event to put this code in