Your first payment timing depends on when your policy starts, relative to your chosen payment date (the 1st, 15th or 28th of the month).
If your policy starts on your payment date
If your policy start date matches your chosen payment date, there's no partial period to worry about — you'll simply be charged your full quoted premium on that date, and then again each month going forward.
If your policy starts between payment dates
If there's a gap of more than 5 days between your policy start date and your next payment date, you'll be charged a smaller, prorated amount straight away (covering just the days until your next payment date). Your first full monthly payment will then follow on your usual payment date, and it'll match the price you were quoted.
If your policy starts close to your payment date
If the gap between your start date and your next payment date is 5 days or fewer, we don't take a separate small payment for those few days. Instead, that prorated amount is added to your next full payment — so your very first payment will be slightly higher than the price you were quoted, covering both the prorated days and the following month in one go. Every payment after that will be at your quoted price.
Example
If you signed up on the 28th of the month, with a monthly payment date of the 1st:
28th (signup): no charge taken yet
1st (following month): a slightly higher payment is taken, made up of the prorated amount for those few days plus your full month's premium
Ongoing: your normal quoted premium each month from then on