Ghost/core/server/data/meta/published_date.js

function getPublishedDate(data) {
    var context = data.context ? data.context[0] : null;

    context = context === 'amp' ? 'post' : context;

    if (data[context] && data[context].published_at) {
        return new Date(data[context].published_at).toISOString();
    }
    return null;
}

module.exports = getPublishedDate;