protected Date getSignatureDate(int offsetInSeconds) {
        return new Date(System.currentTimeMillis() - offsetInSeconds*1000);
    }
Returns the current time minus the given offset in seconds.
The intent is to adjust the current time in the running JVM to the
corresponding wall clock time at AWS for request signing purposes.

@param offsetInSeconds
           offset in seconds
aws-java-sdk-core/src/main/java/com/amazonaws/auth/AbstractAWSSigner.java