The £15.99 first month membership offer is worked out on a pro rata basis and has two direct debit payment date options - the first or fifteenth of every month.
The date you sign up and choose to pay your direct debit determines the amount you pay for your first month (based on a full month at £15.99).
For example, signing up on January 8th and opting to pay your direct debit on February 1st means you will receive 23 days membership at a pro rata fee of £19.72.
However, if you were to sign up on January 8th and choose to pay your first direct debit on February 15th, you will receive 38 days membership at a pro rata fee of £19.72.
After the first month, the usual membership fee of £29.99 resumes and will be paid for on your selected direct debit date.