Hi Srikanth,
No need of 31.12.9999 and it depends upon the quota applicable for the employee let the date be selected by the employee and if its 31.12.9999 it would be a manual activity for the HR admin to delimit.
In case of long term sick leave there may be some business rules where LOP may be applicable. Its just my opinion not to use 31.12.9999 as everywhere in common there would be a quota associated with sick leave.
Regards,
Mithun K