Custom record field

I want to create a custom record field that converts a string timestamp field to date or datetime just to use it in user filters. I chose the real-time option but it seems something is wrong with that because when I tried with sync-time evaluation it works. This is the code: 

datetime(
left(ri!stringTimestamp, 4),
mid(ri!stringTimestamp, 6, 2),
mid(ri!stringTimestamp, 9, 2))

instead of ri!stringTimestamp I am using the respective record field