My email had a large bolded section specifically addressing submission deadlines. Here it is verbatim:
SUBMISSION DEADLINES:
For “regular M.D.” applicants the deadline for submitting the secondary application is one month from the date on which you received this invitation (e.g. if you receive the email on August 8, your deadline is September 8)
For applicants to the MD/PhD program, the deadline is two week from the date on which you received this invitation (e.g. if you receive the email on August 8, your deadline is August 22)
Note that these deadlines do not apply to submission of your letters of recommendation. The deadline only applies to things for which you are directly responsible – your secondary application and the secondary application fee payment.