1. Umwandeln in ein ISO-Datum:

date(substr(digits(mydate), 1, 4) || '-' || substr(digits(mydate), 5, 2) || '-' || substr(digits(mydate), 7, 2))

Umrechnen in Tage:

days(
date(substr(digits(mydate), 1, 4) || '-' || substr(digits(mydate), 5, 2) || '-' || substr(digits(mydate), 7, 2))
)

Differenz also:

days(
date(substr(digits(mydate), 1, 4) || '-' || substr(digits(mydate), 5, 2) || '-' || substr(digits(mydate), 7, 2))
) - days(
date(substr(digits(mydate2), 1, 4) || '-' || substr(digits(mydate2), 5, 2) || '-' || substr(digits(mydate2), 7, 2))
)