2.52 knots = 4.66704 kilometers per hour
and 2.12 knots = 3.92624 kilometers per hour
Therefore 2.52 knots is faster.
You have asked for a difference in time in your question, when really the only difference is speed. The difference in speed for this is:
(4.67 / 60) - (3.93 / 60) = 0.012 kilometers per minute difference.
