Very simply you can put a link through RSLinx OPC to the tags in the controller.
suppose you have the RTC data in a UDT
RTC.Year
RTC.Month
RTC.Day
RTC.Hour
RTC.Minute
RTC.Second
RTC.uSEc
Then in a cell in Excel you can put the formula...
=RSLinx|Target!"RTC.Hour"
Note that Excel does not report if this link is broken - to do this properly you need to write VB code.....