Datepicker in drupal is rendered as separate select elements for day, month and year by default which makes it difficult to set a default value if we want to set it to some string value, eg.
+2 months, etc. This page is to check how we can set default value for datepicker using a simple jQuery.
Get string from PHP
Consider you have the above string value. If you are sending this value from a php code, convert it to a date format using