Error in setup Xcode 4.2.1?

Question

I have a problem when I try to set up Xcode . shown in image

Here's what's in my install.log file:

Apr  1 01:29:04 Bashars-MacBook-Pro Installer[420]: Create temporary directory "/var/folders/_4/pnfcjvx94hbgzfgsyy387b9w0000gn/T//Install.420mLpMwf"
Apr  1 01:29:04 Bashars-MacBook-Pro Installer[420]: IFPKInstallElement (29 packages)
Apr  1 01:29:04 Bashars-MacBook-Pro Installer[420]: Using authorization level of root for IFPKInstallElement
Apr  1 01:29:04 Bashars-MacBook-Pro Installer[420]: Failed install preflight: Error Domain=PKInstallErrorDomain Code=102 "The package “DeveloperTools.pkg” is untrusted." UserInfo=0x7fb8526d33d0 {NSLocalizedDescription=The package “DeveloperTools.pkg” is untrusted., NSURL=../Packages/DeveloperTools.pkg -- file://localhost/Users/bashar/Desktop/Programs/Xcode%204.2.1%20iOS%205/Install%20Xcode.app/Contents/Resources/Xcode.mpkg/, PKInstallPackageIdentifier=com.apple.pkg.DeveloperToolsLeo, NSUnderlyingError=0x7fb8526b61e0 "The operation couldn’t be completed. CSSMERR_TP_CERT_EXPIRED"}
Apr  1 01:29:04 Bashars-MacBook-Pro Installer[420]: Install failed: The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance.
Apr  1 01:29:04 Bashars-MacBook-Pro Installer[420]: IFDInstallController 50D06C30 state = 7
Apr  1 01:29:04 Bashars-MacBook-Pro Installer[420]: Displaying 'Install Failed' UI.
Apr  1 01:29:04 Bashars-MacBook-Pro Installer[420]: 'Install Failed' UI displayed message:'An unknown installation error occurred.

The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance.'.

Answer 1

The certificate used to sign it has expired. Set your date to around when 4.2.1 was released and then install it.