There is a free extension for Firefox and Chrome that lets you send emails at a specific time in Gmail: "Right Inbox".
I'm using it in Chrome with multiple Gmail accounts and it integrates seamlessly and smoothly. I can schedule to send an email in 1, 2, or 4 hours, or at a specific date and time. This is particularly helpful when sending reminders and newsletters.
Just go to the Chrome Web Store or to the Right Inbox website and install the extension. Enjoy!