Chess Time uses ELO for calculating ratings. Below are the categories for the ELO ranges.
Grand Master = 2400
Master = 2200 -> 2399
Expert = 2000 -> 2199
Class A = 1800 -> 1999
Class B = 1600 -> 1799
Class C = 1400 -> 1599
Class D = 1200 -> 1399
Class E = 1000 -> 1199
Class F = 800 -> 999
Class G = 600 -> 799
Class H = 400 -> 599
Class I = 200 -> 399
Class J = lower than 200