Customize the time zone in a workbook's settings and in the NOW and TODAY functions.
When I use the NOW() function in Excel for desktop, it correctly displays US Central time, which is the time zone my computer is set to. When I use it in a new, blank workbook created in Excel online, it displays it in US Pacific time (my Outlook online settings are set to Central time). When I use it in an online Excel workbook that's shared with my work colleagues in the UK, it displays it in GMT. This seems like it may be a bug that needs to be fixed, but I would also add two suggestions:
Within the settings for an individual workbook saved in the cloud, let users toggle between two options: (a) display all date/time stamps according to each user's localized date/time (based on their computer or mobile settings) or (b) display all date/time stamps according to a single time zone selected from a dropdown list so that all users will see the same date/time regardless of their geographical location.
Add a time zone argument to functions such as NOW() and TODAY(). In the spreadsheet my team uses, we need certain formulas to return a date/time value in GMT and other formulas to return a date/time value in US Central time for all users, even if they don't live in those time zones. Adjusting the value by adding or subtracting a certain number of hours would be impractical both because of daylight saving time and because of the confusion over which time zone the spreadsheet is actually going by (see my first paragraph above). So, for example, typing =NOW() would display the date/time of the localized time zone or the workbook's default time zone. Typing something like =NOW(CT) would always display the Central time zone.