The answer depends on why you want to do this and how you have the Date/Time setup on your web page.
Usually any display of Date and Time in a webpage is obtained from the computer of the website browser ie their local time - you can introduce offsets into this - see last paragraph.
If you are using the hosting server date/time then it may be a static date/time display or dynamic continually updating from the server, the server may be in another time zone well away from yours
There are many Javascript Date/Time clocks available on the Internet of varying complexity and options - eg Time zone correction