Date format and expressions in TI for time dimension

[dagland]

Hi All,
Hoping you can help me.
I am trying to set up a TI process to import data into a cube and am having an issue with the date.
In my data I have a date field that shows the date as yyyymmdd (eg 20150511), I am using an expression in the process to convert this to date format that TM1 recognises as a time dimension (I have set up a time dimension using the populate time dimension tool).
The expression I am using is:
vtheDate = Subst(theDateNumber,1,4)|'-'|Subst(theDateNumber,5,2)|'-'|Subst(theDateNumber,7,2);
But this doesn't seem to be working, when I run the process it brings in the result of the expression but not using the date dimensions that exist (my date dimension covers the time period of the data)
Any advice on what I am doing wrong would be appreciated

Thanks
Danielle