The IB Chat dude said a calendar month is EST based, but he added words like "generally", "trading day", etc, which can be confusing, but whatever...
Actually, when I just tried to withdraw money on IBKR website now (prior to EST-based calendar month) it shows below (which is what I wanted to know): Fee withdrawal schedules are based on Eastern Standard Time (EST). You have exceeded the maximum allowed free withdrawals per month and a fee will be charged for each additional withdrawal request submitted this month. Should you wish to proceed with the transfer at this time, click on Continue. By doing so, you acknowledge and agree to the monthly withdrawal fee. Alternatively, you may wait until the first day of the next month, as denoted in EST, to request a withdrawal in order to avoid the withdrawal fee.