With a Catalog or File System search, the ArcCatalog Search dialog will only find items with FGDC metadata when searching by date. Dates in ISO metadata documents are not searched.
Beginning with ArcCatalog 9.1, searching an ArcIMS Metadata Service by date will find both FGDC and ISO documents.
For all Time Period of Content searches, only dates defined as a single date are evaluated when searching FGDC metadata documents. If the FGDC document contains a time period defined by multiple dates or by a date range, those time periods are not searched and the item can not be found.