Sure. Pipe it through g/awk:
awk '{
if (NR == 1) { print "---"; }
if ($1 == "date:") {
printf("%s %sT%s-07:00\n", $1, $2, $3);
} else {
print $0;
}
}' < yourFile > newFile
This example assumes that your timezone is 7 hours behind UTC.