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;
    }
  }
}