.card-item{                     /* already in the template */
    display:block;
    position:relative;
}

/* Date badge */
.date-badge{
    position:absolute;
    top:.75rem;
    left:.75rem;
    z-index:5;                 /* ←  add this  */
    background:#fff;
    color:#000;
    text-align:center;
    padding:.25rem .5rem;
    border-radius:4px;
    font-weight:600;
    line-height:1;
    box-shadow:0 1px 3px rgba(0,0,0,.15);
}
.date-badge .day  {display:block;font-size:1.25rem;margin:0}
.date-badge .month{display:block;font-size:.65rem;text-transform:uppercase;letter-spacing:.03em;margin:0}