The announcement date is the release date of such information to the market. IBES has two variables related to the time of the earnings announcement: ANNDATS and ACTDATS. The former refers to the time when the company announced earnings (according to the business newswires), whereas the latter refers to the date when the information was entered into the IBES database.
On the other hand, Compustat has a report date of quarterly earning which is available only in Compustat quarterly data ( a variable is RDQ and its definition by Compustat is "This descriptor represents the date in which quarterly earnings and earnings per share figures are first publicly reported in the various news media.").
We checked many instances, and it appears that ANNDATS and RDQE are the same, but in case you notice any difference between them, I recommend that you use RDQ as a more reliable "announcement date".
A Paper by Livnat and Mendenhall (JAR, 2006) "Comparing the Post-Earnings Announcement Drift for Surprises Calculated from Analyst and Time Series Forecasts" provides a nice discussion of the differences between Compustat and IBES data.
Recently, there have been several papers reporting issues with IBES announcement dates. For more details about these findings, please, check a WRDS research presentation at https://wrds-web.wharton.upenn.edu/wrds/E-Learning/_007Course%20Materials.cfm entitled "Overview of Thomson Reuters".