This will give you the difference in record counts, not the number of records added. For example:

  t +  1: add  5 rows
  t + 10: add  5 rows
  t + 20: del 20 rows
  t + 30: add  5 rows
  t + 50: add 10 rows
[download]

Your code would give an answer of 5 if $ct1 was obtained at time t. moritz's trigger answer would give a more accurate answer if you actually need the could count [1] of items added, and not just the difference in the row counts.

Updates:

1. Fixed spelling mistakes.