Every day we check account's numbers and store it in our database. Usually we perform it at midnight GMT.
Also for accounts that are our clients interested in we check numbers more often (every hour). In this case you will see a row with exact date and time.
Checking period is different for different accounts. For popular accounts (more than 10-20k followers) it's every day, for less popular accounts we check it less frequent. But you can always ask us to increase frequency and check your account every day.