String field to Date field

An FME date has form yyyymmdd (also described as %Y%m%d), so it needs a month part and a day part. If a month and day are not provided, then I don't think the DateTimeConverter can default it to something.

I'd use an AttributeCreator, with a Conditional Value to say, if the given 'date' matches regex ^[0-9]{4}$, then add a '0101' 1 January to the end of the string like: @Value(Current Year)0101

If there are more date formats used other than the four digit year, then you'll need to add a different rule for each case.

Once the the numbers are in that yyyymmdd form, then the date functions, writers etc will work with it as if it is a date.

Rather than convert the string to a date, you can convert the current year date to a value of just the year and use that in the test filter, e.g.