The inferred country is already stored. It would save us all a lot of time if Marketo already auto-populates a field called 'Inferred Timezone'. The positive or negative number compared to GMT should be derived from you locale setting. This should be very easy to build. Now we would only need a checkbox for the wait step options when you select a specific time: 'Send according to lead's local time'. This will add or deduct the inferred timzone number value from the time you select (based on your instance's locale). The great benefit from this is that your mailing to x-thousand of people is not send in one go, but throttled throughout the day. This has multiple system benefits of course.
Related ideas - Wait Until Step Using Local Time: http://community.marketo.com/MarketoIdeaDetail?id=08750000000Inr5AAC - Different time zone set up for email sending within one campaign (merged): http://community.marketo.com/MarketoIdeaDetail?id=08750000000Hss6AAC - Sending Emails by Time Zone: http://community.marketo.com/MarketoIdeaDetail?id=08750000000IY98AAG