Date Diff Giving Null Values

Hi,

I want to caclulate the no of days an employee has been with the company so if a person has not left the company and does not have a leaving Date it should take the current date.

We have tried the following formula mentioned below but gives null result. Any pointers to solve this would help.

firstOne

vba!datediff("d",[Employee].[Joining Date].currentmember.member_value,

iif(isempty({}),

tail(descendants([Date].[FiscalHierarchy].currentmember),1).item(0).member_Value,

[Employee].[Leaving Date].currentmember.member_value))

Second

iif(

[Employee].[Leaving Date].[Leaving Date].currentmember.member_Value=NULL ,

tail(descendants([DateDay].[WeekHierarchy].currentmember),1).item(0).member_Value,

[Employee].[Leaving Date].currentmember.member_value

)

Third 

vba!datediff("d",[Employee].[Joining Date].currentmember.member_value,

iif([Employee].[Leaving Date].[Leaving Date].currentmember.member_Value=NULL ,

tail(descendants([DateDay].[WeekHierarchy].currentmember),1).item(0).member_Value,

[Employee].[Leaving Date].[Leaving Date].currentmember.member_value))