Get value from HTML Tag (span)

With javascript you can do the following

var s = document.getElementById('test').innerHTML;
var d = document.getElementById('test').textContent || document.getElementById('test').innerText;

With JQuery

var x = $('#test').text();
var y = $('#test').html();

If you want to submit the value with the form and retrieve it on the server - can't

yes, I want to submit that value with php. I want to get date value from span in bootstrap-datepicker and submit that value to php.
https://bootstrap-datepicker.readthedocs.org/en/latest/options.html

can I transfer value from span to hidden textbox when have any change on span? if can, how to do it?

can I transfer value from span to hidden textbox when have any change on span?

I don't understand - <span> is not an input element - it contains static text so not sure what you mean when you say "have any change on span"?

I use https://bootstrap-datepicker.readthedocs.org/en/latest/options.html 
when I pick any date, it will give value to span. so I want to use that value in span with form.

when I pick any date, it will give value to span

A) How?
B) Why?

Why are you assigning a date to a span and not an input element?

I not assign it myself. I use date-picker from this web https://bootstrap-datepicker.readthedocs.org/en/latest/options.html 
and I saw when I pick date, the value was change in span.