It is possible to configure Thunderbird to include the date and time of the mail being replied to. I.e. to replace

Foo wrote:
> Hello Bar!

with

On 02/05/2009 11:27 AM, Foo wrote:
> Hello Bar!

It isn’t done through the configuration dialogs, rather it is configured through user.js (which might have to be created). All one essentially has to do is create a file name user.js containing

user_pref("mailnews.reply_header_type", 2);

in the Thunderbird profile directory.