Record cell change with time stamp

[Migs24]

Hi experts. I have a macro when I press, retrieves headcount from an external program by formulas display it on f50 and g50. I want to have vba record any change in values of the range and then pasted on a time table based on 30 minutes period. Eg. 9:01,9:02...9:30 or 10:31..10:32....11:00 (disregard hour, it is per 30 minutes, rinse repeat). If I press the retrieve macro, I need the vba to paste new values to the minute it was run.. ex. If I run retrieve macro by 5:03, the macro will paste the two cell values on the 3rd minute, run again at 5:05, record values in 5th minute. . I have a table 1st to 30th minute on colmun BB1 to BB30 (00:01, 00:02, 00:03). Sorry can't post image file is on office.