It is equivalent to approximately 0.31689 Earth Years due to the fact that a Slab day is 15.74% longer than an Earth day (see Time Decimalisation).
Even though certain events are triggered in cycles and places such as Zone61 refer to citizen's ages in cycles, almost no-one observes cycle-birthdays, preferring to convert their ages to Earth years and celebrate Earth-style annual events instead. This creates no end of confusion.
An Earth year is 3.1556952 cykes which means that birthdays only fall on the same day of a cyke in a complicated pattern of 45, 122 and 167 year intervals (due to the rounding requirements of having birthdays last one complete SlabDay).
This means that everyone relies on Sis to provide advance warnings of upcoming birthdays to each citizen and all of the citizen's extensive SocNet. This phenomena led directly to the GiftCrash and resulted in the banning of Recursive Gifting.