Okay, so just in case anyone needs help on the same thing I did… here is my solution. Instead of a php/wordpress solution, I used javascript. Id love to see the php/wordpress solution if someone wants to post it.
<?php
// Admin Custom JS
add_action('admin_head', 'admin_custom_js');
function admin_custom_js() {
?>
<script>
// update date (month) taxonomy term on change of start date acf field
jQuery(function($){
var $startDateInput = $('div[data-name="start_date"] input');
if ($startDateInput.length) {
$startDateInput.on('change',function(){
var val = $(this).val();
var moList = ["January","February","March","April","May","June","July","August","September","October","November","December"];
if (val === ''){
} else {
var moNum = val.substr(0,2);
moNum = moNum.replace('0','');
moNum = Number(moNum) - 1;
var moName = moList[moNum];
var termVal = $('#datechecklist').find('label:contains("'+moName+'")').find('input').val();
var msg = 'Start date month set to '+moName;
$('#datechecklist input').prop('checked',false);
$('input[value="'+termVal+'"]').prop('checked',true);
$('div[data-name="start_date"] .acf-label p').remove();
$('div[data-name="start_date"] label').after('<p class="description" style="font-weight:bold;color: #0a81d2">'+msg+'</p>');
}
});
}
});
</script>
<?php
}