Date() in the JavaScriptCommand not working properly

#pragma once
// timestamp: Sun Nov 15 2392
#define TIMESTAMP 13344641115

import qbs.TextFile

CppApplication {
    name: "TestApp"
    files: "main.cpp"

    cpp.includePaths: [
        product.destinationDirectory,   // for timestamp
    ]

    // generate timestamp header
    Rule {
        alwaysRun: true
        multiplex: true
        Artifact {
            filePath: "timestamp.h"
            fileTags: "hpp"
        }
        prepare: {
            var cmd = new JavaScriptCommand();
            cmd.description = "generating " + output.fileName;
            cmd.sourceCode = function() {
                var f = new TextFile(output.filePath, TextFile.WriteOnly);
                const d = new Date();
                const utc = Math.floor(d.getTime() / 1000);  // in seconds
                try {
                    f.writeLine("#pragma once");
                    f.writeLine("// timestamp: " + d.toDateString())
                    f.writeLine("#define TIMESTAMP " + utc)
                } finally {
                    f.close();
                }
            };
            return [cmd];
        }
    }
}