JSFiddle - React, Tailwind, and code Playground
HTML
<input id="user_join_date">
<input id="user_probation_end_date">
CoffeeScript
joinDate = $('#user_join_date')
joinDate.blur ->
m=joinDate.val()
console.log m
splitdate = m.split('/')
probationDate = new Date(splitdate[2], splitdate[0] - 1, splitdate[1])
console.log probationDate
myVar = new Date(probationDate.setMonth probationDate.getMonth())
$('#user_probation_end_date').val(myVar)