CSS Grid - IE 11 Overlap

Try using this PostCSS autoprefixer:

https://autoprefixer.github.io/

You need to position every element by hand using -ms-grid-row and -ms-grid-column

.calendar > div:nth-of-type(1){
    -ms-grid-row: 1;
    -ms-grid-column: 1;
}

.calendar > div:nth-of-type(2){
    -ms-grid-row: 1;
    -ms-grid-column: 2;
}