More flexible Frontmater parser

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.