To some extent it is very easy. The sending time and date information
is added by your email client. Change the timezone on your computer
and that information changes. It might be necessary to restart you
mailclient to make it see these changes.
However it is a little more complicated than that, as the mailservers
tend to add that information themselves as well in their 'Received'
headers and your outgoing mail server is generally in the same
timezone as you.
You can use a webbased mail service - where you may be able to
configure your local timezone. Although many of those services use
your local timezone, so changing the details on your computer might be
needed there too.
In summary, doing what you want should be able to be achieved by
simply adjusting the settings on your local computer when you send an
email. However as other servers also put a timestamp on the message
that might not be enough to achieve what you want.
That said, the 'Date' information is a header, and is created entirely
by the email client - if you wanted to invest some time in programming
or research it should be possible to send email with no Date header at
all, or a totally customisable one. However this is not generally
possible with standard email clients. Also, mail servers will still
generally add timezone info in their 'Received' headers.
Additionally, you face a larger problem if you are seriously concerned
about masking your geographic locatation - the IP Address. All
outgoing emails you send will include information about your IP
Address, which can normally be geo-located to at least a country level
(if not city and state). This information is much harder to conceal.
What you are trying to do may be possible, but would probably require
access to offshore servers, web-based email systems, and maybe some
sort of anonymising service.