wagtail/client/scss/components/_human-readable-date.scss

// Displays 'timesince' formatted date with full date on hover
.human-readable-date {
    overflow: hidden;
    display: block;
    position: relative;

    &:before {
        position: absolute;
        display: none;
        content: attr(title);
    }

    &:hover {
        visibility: hidden;

        &:before {
            visibility: visible;
            display: block;
        }
    }
}