"First" what?
You logic is not actually clear as the MBR=PLAT is the "first" MBR value for the date 31-Jan-18. So it really doesn't appear that "date" is that important from the very small sample provided.
You may need to show an example, which can be made up, but provides more examples that would clarify the rule(s) involved.
I can write code that would solve this particular example but would be very unlikely to extend to other combinations of dates or MBR values correctly.