Site generation not resolving git.commit.time

I'm using git-commit-id-maven-plugin to set project.build.outputTimestamp as suggested in the Maven Guide Configuring for Reproducible Builds. I've included the plugin in my pom and including the following property definition:

<project.build.outputTimestamp>${git.commit.time}</project.build.outputTimestamp>

When I generate the site, the plugin fails with the message:

site failed: Invalid project.build.outputTimestamp value '${git.commit.time}

This configuration works properly in the maven-jar-plugin, so I did some investigation. The jar plugin is getting the outputTimestamp as a mojo parameter:

@Parameter( defaultValue = "${project.build.outputTimestamp}" )
private String outputTimestamp;

This works properly, with outputTimestamp set to the last commit time.

In the site-plugin code, the outputTimestamp is being pulled from the project properties:

String outputTimestamp = p.getProperties().getProperty("project.build.outputTimestamp");

In the case the property hasn't been resolved and the returned string is ${git.commit.time}.