scripts: Offset Date header fields to support sorting by date

Joe Perches wrote:
> Also, when you send these patch series, please use
> an email delay of at least 1 second between each
> entry in the series as the threading is otherwise
> poor in various email clients when sorting by time.

Since we generate all Date headers in advance, we don't actually need
to sleep.  Instead, use the current time for the first message in a
series and add 1 second for each message after that.