Hi Kishore,
I have never used uIS_SetValue with validity dates on an account privilege, only on normal privileges. Only workaround I could suggest is using toIdentityStore pass instead of uIS_SetValue. My add member tasks are always toIdentityStore passes as you have repository and just need simple syntax:
MSKEYVALUE=%MSKEYVALUE%
changetype=modify
MXREF_MX_PRIVILEGE={A}<PRIV:%$rep.NAME%:ONLY>
What version are your running on?
Thanks
Norman