Remove Duplicates based on Date/Time

Hi,

I have column A (Read Time), Column B (Employee ID). I want to delete duplicate employee ID's that were read under 30 seconds of time and keep the oldest time. I want a flag in column C to display "D" if the Employee ID had duplicates.

example:
A4 and A5 are within 30 seconds of each other and the Employee ID is a duplicate. I want to delete row A5 and display a "D" in Column C4.

Read Time	Employee ID	Flag
8/10/18 17:34:02	28113485
8/10/18 17:34:53	28113485
8/11/18 18:34:05	28113485	D
8/11/18 18:34:12	28113485
8/11/18 17:34:53	28113491
8/10/18 17:20:37	28479675

Any Help would be greatly appreciated.