For date information, such as date of birth or date of diagnosis, SEER only collects month and year. The month of birth is not available for research use because of confidentiality concerns. We calculated a months-since-last-birthday variable, which can be used in conjunction with the age in years at diagnosis to derive the age in months at diagnosis.

These variables are calculated as follows:

  1. Birth date in months = (year birth * 12) + month birth, and
    Diagnosis date in months = (year diagnosis * 12) + month diagnosis.
  2. Age in months = diagnosis date in months – birth date in months.

When the birth month and diagnosis month are the same, our estimate of age in months at diagnosis is as accurate as if day information were used in the calculation. However, when the months differ, our calculation is not accurate. For example, for a person with a January birth month and a February diagnosis month, the calculated months since last birthday would be one month. But in reality, it could be anywhere from 1 day to 59 days (assuming 30 days in every month). Therefore, in any calculated month after last birthday, about 50% of patients are in fact one month younger (assuming equal distribution of patients by birth month and diagnosis month). In aggregate, the number of patients with zero month is underestimated by 50% and the number of patients with 11 months is overestimated by 50%. Caution should be exercised in using this variable, given the limitation.

The database is available in the frequency and case listing sessions in SEER*Stat for the November 2018 data submission.

How to Access the Months Since Last Birthday Database

In order to access the custom database, you must already have access to SEER research data through SEER*Stat in client-server mode.

  • If you do not have access to SEER data in client-server mode, first request Access to the SEER data.
  • If you already have access, send an email to to request access to the Months Since Last Birthday Database.
    • Include your SEER*Stat username.
    • Since access to the Months Since Last Birthday Database includes treatment data, a Data Use Agreement for SEER Radiation Therapy and Chemotherapy Information is required regardless of whether you intend to use treatment fields. If you have not already completed this DUA for the current submission, please complete and return it by email.
    • Add a brief description of your project and research goals in the email, including the types of analyses or statistics you will use, including how you would use treatment data, if you intend to use it.
    • Additionally, if you require access to the standard radiation/chemotherapy databases, please include this request in your email.