get client date&time to webserver

hi together,

I want to make a webserver hosting a captive portal, which is working so far.
As the server will be operated with solar power, there might be also periods of black-outs. To log the visits with the correct time and date, I tried to get the client's date and time via javascript.
But here begins my problem, I'm really unexperienced with javascript. I already found some snippets which can display time and date on the webpage. But I have no clue how to send it to the host.
On the initial webpage there is a form, which the client has to fill an send to the host on a button press, that's working already.
Ideally, the date&time will be sent once the webpage is loaded, but I'm also fine if it would be sent with or after the button press.

Can anybody help me how to solve this?

rgds purehunter

Why not use NTP?

there is no internet connection there...