I just ran into this problem when trying to update the application, but my problem was not synchronized out of sync ( ntpdate -q time.apple.com showed a maximum offset of 0.022479).
I found a thread hint on one of the Apple discussion forums that the problem might be related to certificate status verification settings. Modifications in this area solved the problem for me. In the Keychain Access application, after choosing the menu item Access to the keychain > Settings> and going to the Certificates tab (close this settings window when you made any changes), I eventually changed both OCSP and CRL parameters: "Require if Cert Indicates "(for me it seemed useful," strict ")," Best attempt "(Priority remained" Demand "Both").
It appears that the options “Require certificates” and “Require for all certificates” were generally not available (I initially installed them back on a Tiger machine that was ported to this Snow Leopard machine). I found that you can hold the option by clicking on the pop-up button (aka drop-down list) to enable and select these options. I can go back and try to determine which setting (OSCP or CRL) was strictly necessary.