Formula step

[cnv_Ben]

I have an input stream with a field named "DATE_STRING" that is a fully-qualified timestamp with a timezone offset. I want to "clip out" the first 10 characters which would return something like "2012-05-01". I used a Formula written as DATEVALUE(LEFT("client_dt_string";10)). When I choose "Check seleted step(s)" Spoon does not indicate any error. When I run my transform however, Spoon fails and the error log includes:

2012/05/01 17:31:49 - Formula.0 - ERROR (version 4.2.1-stable, build 15952 from 2011-10-25 15.27.10 by buildguy) : Unexpected error
2012/05/01 17:31:49 - Formula.0 - ERROR (version 4.2.1-stable, build 15952 from 2011-10-25 15.27.10 by buildguy) : org.pentaho.di.core.exception.KettleValueException:
2012/05/01 17:31:49 - Formula.0 - ERROR (version 4.2.1-stable, build 15952 from 2011-10-25 15.27.10 by buildguy) : org.pentaho.reporting.libraries.formula.typing.TypeConversionException: LibFormulaErrorValue{errorCode=502, errorMessage=Invalid Argument}
2012/05/01 17:31:49 - Formula.0 - ERROR (version 4.2.1-stable, build 15952 from 2011-10-25 15.27.10 by buildguy) : LibFormulaErrorValue{errorCode=502, errorMessage=Invalid Argument}

So, it appears I have written my formula incorrectly. Can anybody help me out with the correct syntax for this formula?
Thanks!

[Taqua]

Fields are referenced with [fieldname]. You are trying to substring a static string named "client_dt_string", which always result in the string "client_dt" which is not exactly a parseable date.