Thank you for your reply.
My system clock is accurate.
Downloaded the LibreOffice 4.1.5 installer and help files from the “LibreOffice Stable” webpage at https://www.libreoffice.org/download/libreoffice-stable/?type=win-x86&lang=en-US&version=4.1.5.
Successfully verified digital signatures for different programs from a different vendor downloaded less than an hour before the LibreOffice files.
The “Certificate Information” for both LibreOffice files states the following.
"This certificate has expired or is not yet valid.
Issued to: The Document Foundation
Issued by: StartCom Class 2 Primary Intermediate Object CA
Valid from 2012-04-10 to 2014-04-11"
I downloaded and checked the digital signature for both files on 2014-04-28, so the certificates for both expired before I downloaded them.