By average, the days in a month is averaged 30.4, rounded to 30 (365 days a year divided by 12 months in a year). Ideally, you should get 900 total gems on VIP on the months having 30 days, 930 gems total gems on VIP on the months having 31 days, and 840 gems in February (870 gems on a leap year).
But the thing is, the monthly VIP is very fixed. The VIP lasts only for 30 days (30-day pass) or 365 days (365-day pass), depending on which you purchased. That means that the Pass is only good for 30 days. Thus, if you purchased VIP on January 2 (first day claim), your last day to claim should be January 31, and since a day last for 24 hours, it has extended to the first of February, as in your case.
To be able to save (if you see yourself playing for at least 1 year), purchasing the 365-day pass is the way to go since it is cheaper in 2 ways: (1) you get 5 extra days for free (compare it with buying 12 x 30-day pass and it would only total to 360 days, not 365) and (2) the cost is only for 10 30-day VIP purchases. Player can still play the game with or without VIP.