Jennifer,
You MIGHT be able to call tomorrow, but you will be
safer if you call today. Retroactivity has been allowed, but I think it's about the agent and their disposition that day.
richarddd,
You can sign up and begin your Challenge June 16 and get the status you earn through February 28, 2007.
FewMiles' Challenge page states that as well.
Specifically,
FewMiles says: "Start date - You can choose to start your Challenge on the 1st or 16th date of any month. The start date you choose determines how long your elite status will last. Challenges started in the first half of the calendar year (01Jan - 01Jun) will give you elite status to the end of the current membership year only. Challenges started in the latter half of the calendar year (16Jun - 16Dec) will earn you elite status for the remainder of the current membership year as well as for the following membership year."