$minute = $_REQUEST['minute'];
    $minute = floorminute($minute);
    $second = !isset($_REQUEST['second']) ? 0 : $_REQUEST['second'];
    # set the number of minutes between spectrograms, i.e. minutes of data in a spectrogram (assuming no time overlap)
    $numMins = !isset($_REQUEST['numMins']) ? 60 : $_REQUEST['numMins'];
    if ($minute != $_REQUEST['minute'] || $second > 0) {
        # rounded down
        list($year, $month, $day, $hour, $minute, $secs) = addSeconds($year, $month, $day, $hour, $minute, 0, 600);
        $minute = floorminute($minute);
    }
    # For entry from the form, make sure it has correct number of digits
    $year = mkNdigits($year, 4);
    $month = mkNdigits($month, 2);
    $day = mkNdigits($day, 2);
    $hour = mkNdigits($hour, 2);
    $minute = mkNdigits($minute, 2);
    $sgram = "{$WEBPLOTS}/{$subnet}/{$year}/{$month}/{$day}/" . $year . $month . $day . "T" . $hour . $minute . "00.png";
} else {
    # Get latest spectrogram for this subnet, and then form date/time variables from its filename
    $sgramfiles = recentSpectrograms($subnet, $WEBPLOTS, 1, 30);
    $sgram = $sgramfiles[0];
    list($year, $month, $day, $hour, $minute) = sgramfilename2parts($sgram);
}
# Debugging
if ($debugging == 1) {
    echo "<p>numMins = {$numMins}</p>\n";
    echo "<p>subnet = {$subnet}</p>\n";
    echo "<p>year = {$year}</p>\n";
    echo "<p>month = {$month}</p>\n";
    echo "<p>day = {$day}</p>\n";
    echo "<p>hour = {$hour}</p>\n";
function floorMinute($minute)
{
    $numMins = 10;
    $floorminute = floor($minute / $numMins) * $numMins;
    $floorminute = mkNdigits($floorminute, 2);
    return $floorminute;
}