Time To Expire is calculated as (now - Termination Date or Expiration Date) and and there will be many groups created if used in a perspective.
The Expiration Date is the original ending date of the contract. This date shouldn't change, regardless of whether an RI was modified, sold, etc. and the Termination Date will match the Expiration Date if nothing about the reservation was changed. However, if the reservation has been sold, modified, etc, the Termination Date will reflect the date of those changes.
The "Time to Expire" column is the number of days until the Termination Date and this can change if an RI is modified, sold, etc. and in these cases may not match the Expiration Date.
Time To Expire should not be utilized as a categorizable or filterable attribute as it is ever-changing.