Variable Qualification Error

HI

I have below expression, I am trying to create a variable string type and dimension but it is automatically converting to measure even though that is string. If i try to change as dimension then am getting an error

the qualification of the variable cannot be changed. (ies 10083) (wis 10083).

How to convert this to dimension, this is because of min(udate).

=If Substr(Min([Udate]+[Utime] +[Value]);15;1000)="0" Then "" Else LeftTrim(Substr(Min([Udate]+[Utime] +[Value]);15;1000))