Yes, you can change the billing date for your monthly subscription. On the Ploom website, visit "My Subscription" in your Account Dashboard. Scroll down the page until you reach "Payment Information", click "Edit Billing Date", and follow the outlined steps. 

For the date change to take effect in the current month, you must make the change before your current billing date and choose a date before the end of the current month. For example, if your current billing date is on the 15th of each month and you want to change it to the 20th, you can amend the date up until 11:59 PM on the 14th and the change will take immediate effect. If your monthly billing date has already passed for the current month, or if you want to change your billing date so it’s earlier in the month, your change will take effect the following month. For example, if it is the 14th of the month and your current billing date is the 15th, but you want to change it to the 10th, you will still be billed on the 15th of this month, and it will change to the 10th of the following month.