Time zone auto detection and changes in account pages

[victor]

document: Additional tasks for ITCPay.docx

Now you have to make "Time zone" dynamic in ITCPay site. Each time an user log in to his account, his time zone will get reset automatically. For example, if the user log in to his account from a different time zone, his ITCPay account's time zone will automatically be updated upon his login.
Apart from logging in to the account, when the user is already logged in, each time he clicks on any link in his account, his time zone will get reset automatically. For example, when an user is already logged in, if he visits to a place that has a different time zone and then clicks on any link in his account, his time zone will automatically get reset.
Please enable this feature.
After you enable this feature, you can remove time zone field from personal account sign up page and business account sign up page as we do not need to ask users for this information anymore. And you can remove time zone changing option from "My account" page as well.
Though we remove time zone from personal account sign up page, business account sign up page and "My account" page, a time zone will always be associated with the user's account. This is because when an user receives a payment in his account, time of receiving payment should appear in the "Date and table" column of the "Transaction table" and in the "Details" page of that transaction in the "Transaction table". For calculating this time, a time zone is always need to be associated with the account.
When an user sends money, the system will detect the user's time zone upon his click on the "Send money" button and the time of sending money should be calculated based on that time zone.
But when an user receives money, time of receiving money needs to be calculated as well. For this calculation, a time zone always need to be associated with the user's account. So, our system will automatically update the user's time zone each time he log in to his account. And furthermore, even when an user is logged in to his account, our system will automatically update his time zone each time he clicks on any link in his account.
Please note that the time zone associated with the user's account is used to calculate the date and time that appears in the "Date and time" column of the "Transaction table" and to calculate the date and time that appears in the "Details" page of any transaction in the "Transaction table".
Also note that the date and time that appears in the "Transaction table" in the sender's account may not be same as the date and time that appears in the "Transaction table" in the receiver's account for the same transaction. Because sender's account and receiver's account may have different time zones. Our system calculates the time in the sender's account and time in the receiver's account separately. So even for same transaction, the date and time that appears in the sender's "Transaction table" and in the receiver's "Transaction table" may not be same.
But I am not sure how to detect an user's time zone accurately. Will it be IP address based? How reliable is IP address based time zone? Some countries like United States, Russia have multiple time zones.
Also we can detect the time of the user's computer. But how reliable is it? What if the user's computer's time is not correct.
So how do online services like Gmail detect the accurate time zone of the users?
You should use the accurate method of time zone detection.

[victor]

To detect time zone we can use ​JavaScript

But updating each time user clics on a link or submits a form is not really a coorect way of handling client time zone.
What if client's PS does have wrong time zone configured, or he lives in one time zone but wants to see dates in another ?

I'd propose to leave time zone setting in profiles and sync it with the one browser reports only on client log in. Plus add a message (under menu) "Your timezone <detected by browser> seems to be diffrent from saved in Your profile <profile timezone>. Do You want to update it [Yes] / [No]"
[Yes] will update timezone and hidethe message
[No] will just hide this message