I'm not going to reinvent the wheel, here is the best answer on when spend counts towards your MQD waiver, with a link to the rules on the AMEX website:
Originally Posted by
Zorak
this info can be found in the T&C for the card:
https://www.americanexpress.com/us/c...card/36182-9-0
MQD Waiver
If the Medallion Qualification Dollar (MQD) thresholds are not met for a Qualification Year, you can still qualify for Platinum, Gold and Silver Medallion Status if you have earned the required Medallion Qualification Miles (MQMs) or Medallion Qualification Segments (MQSs) and make at least $25,000 in eligible purchases within that Qualification Year on your Eligible Card. If the MQD threshold is not met for a Qualification Year, you can qualify for Diamond Medallion Status if you have earned the required MQMs or MQSs and make at least $250,000 in eligible purchases within that Qualification Year on your Eligible Card.
A Qualification Year is from January 1 to December 31 of a given year. The close of the Qualification Year is December 31, without regard to the time of the year that the credit card account is opened.
This means that for the first year of Card Membership, the Card Member's eligible purchase period may be less than twelve months.
(bolded the relevant sentence)