We are aware of a new issue today where installing SSDT will fail with the following message "A required certificate is not within its validity period when verifying against the current system clock or the timestamp in the signed file." This is due to an issue in the installer verification code with a SHA256 certificate that expired on 10/6/2013 and we are actively working to update the installer. There are two workarounds:
1) You can install the SQL Server 2012 SP1 components separately and the rerun the SSDTSetup.exe installer. The components necessary are SharedManagementObjects.msi (x86 and x64), SQLDOM.msi (x86 and x64) , TSQLLanguageService.msi (x64), SqlLocalDB.msi (x64), SQLSysClrTypes.msi (x86 and x64), and SSDTDBSvcExternals.msi (located here).
2) You can set your clock back to before 10/6 and then run the SSDTSetup.exe installer
User should not be affected if they are doing an update from a previously installed SSDT as the SQL Server 2012 SP1 MSIs should already be present on the machine.