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You folks really have to stop specifying an explicit date in your patch postings, it's seriously screwing up my patchwork queue.
Alex Riesen's profile photoCyrill Gorcunov's profile photoJunio C Hamano (Gitster)'s profile photoDavid Miller (dokebi)'s profile photo
When you do git send-email it uses the commit date as the email date field by default, it's completely pointless.
AFAICS, send-email will use the current time, not that of commit:
$time = time - scalar $#files;
sub send_message {
    my $date = format_2822_time($time++);
    my $header = "From: $sanitized_sender
Date: $date
+Junio C Hamano ?
+David Miller that's strange, I've been sending a series a couple of hours ago, and received copies
Date: Tue, 26 Jun 2012 12:07:18 +0400
while commits have
Date: Mon, 25 Jun 2012 21:12:04 +0400
+Alex Riesen The timestamps are based on the localtime; we grab a single localtime() and then subtract N seconds from it for N message series, The timestamps are incremented by 1 second for every piece of e-mail to pretend that they came one second apart, so that recipient can sort them by sender timestamp. We even strip out "Date:" header in its input.

But I wouldn't be surprised if an ancient version had a different behaviour. This was a contributor-contributed tool, and contributors tend to want to pee in the snow, so...
I suspect there are people who are using non-default options for one reason or another, or as Junio suggested, perhaps really old tools.