The Library table has two columns that refer to when the repository items were created or last updated:
Generally, the created date will be earlier than the modified date. In some situations, however, the created date may be after the modified date. This can happen in two situations:
When an existing report (A) is modified, and is then subsequently copied into a new report (B). In the Library list, report B has a created date of the day it was created, but its Modified Date is the date reflects the last time report A was changed.
An existing report is exported from one system and imported into another. In the Library list, the reports created date is the date it was imported into the new system, and the modified date is the date it was last modified in the original system.