code:
$ gpsbabel -i gpx -f molochnoje.gpx -x track,speed,merge,start=2009022309,stop=2009022311 -o gpx -F - | gawk '
BEGIN {
        FS = "[><]"
        oldtm = 0
        odo = 0
        max = 0
        fulltime = 0
        firsttime = 1
        deltat = 0
}
/time/ {
        if(firsttime) {
                firsttime = 0;next
        }
        sub("T","\\ ",$3)
        sub("Z","",$3)
        getTime = "date -ud " $3 " +%s"
        getTime | getline tm
        close(getTime)
        if(oldtm > 0) deltat = tm - oldtm
        oldtm = tm
        fulltime += deltat
}
/speed/ {
        if(max < $3) max = $3
        odo += $3 * deltat
}
END {
        printf("Tim: %f\nMax: %f kmph\nOdo: %f km\nAvg: %f kmph\n", fulltime, max * 3.6, odo / 1000, odo * 3.6 / fulltime)
}'
Tim: 7185.000000
Max: 31.420217 kmph
Odo: 12.481229 km
Avg: 6.253643 kmph

code:
<trkpt lat="59.246628284" lon="39.833421707">
  <ele>110.208252</ele>
<time>2009-02-23T11:54:46Z</time>
</trkpt>

code:
/time/ {
        if(firsttime) {
                firsttime = 0;next
        }