Yes, @mnlubke, that is correct.
Let’s assume someone was born on April 22, 1920. For this patient, we randomly generate a date-shift value of 229 days, and shift all of her dates – including her birthday – by this duration. Her date-shifted birthday is now December 7, 1920.
On December 7, 2010, this patient turns 90 years old in date-shifted time. On this day, we shift the patient’s birthday forward by 1 year to December 7, 1921. Now the patient appears to be 89 years old in date-shifted time.
On December 7, 2011, this patient turns 90 years old again in date-shifted time. We repeat the procedure: move the patient’s birthday forward by 1 year to December 7, 1922. Magically, the patient is 89 years old again in date-shifted time.
We continue to shift elderly patients’ dates of birth forward by 1 year annually on their date-shifted birthdays.
The birthdays of all patients under the age of 90, in date-shifted time, are not changed.