Technically, everything under the covers is a string in DataStage and also a Sequential file has nothing but Char data in it. Can't say why exactly it doesn't like your date format off the top of my head, but if you define it as a Char(10) you won't have problems.
How much confidence do you have in the format you are receiving it in? There are things you could do to validate or rearrange incoming "date" fields from a flat file, but your incoming format (as is) is good for ordering or maxing or whatever you may be doing in your Aggregator besides grouping.
Suggestion would be to only declare it as a Date just before it goes into the target - and then ensure it is properly formatted for the particular target. Not everything out there likes YYYY-MM-DD for dates.
EDIT - Ok, now I see why it is happening.
Didn't realize the aggregator is "stupid" as I almost always handle dates as strings through them. Either that or as Timestamps... don't recall having issues with those.