You can query the ReportServer database tables to get this information.
Try something like:
select reportPath = r.Path
, report = r.Name
, created = r.CreationDate
, createdBy = c.UserName
, modified = r.ModifiedDate
, modifiedBy = m.UserName
from Catalog r -- report
inner join Users c on r.CreatedByID = c.UserID -- created
inner join Users m on r.ModifiedByID = m.UserID -- modified
where r.Type = 2 -- report catalog items only
Which returns information on all reports on the server, with created time/user and modified time/user.
You can filter by the datetime values as required.