garmin_timestamp16_to_timestamp

#!/bin/bash
# Maintainer: Gerolf Ziegenhain <gerolf.ziegenhain@gmail.com>
# Algorithm based on: https://stackoverflow.com/questions/57774180/how-to-handle-timestamp-16-in-garmin-devices

gawk '
BEGIN { 
 /* Garmin epoch starts on 1989-12-31 */
 magic=631065600; 
}

/"timestamp"/{
 time_fmt=$2;

 /* Convert to suitable input format for awk*/
 /* 1989 12 31 23 21 12 */
 gsub("[-:T]"," ",time_fmt);

 /* remove " and , */
 t1=substr(time_fmt,2)
 t2=substr(t1,1,length(t1)-2)

 /* Save marker offset time */
 dt_offset=mktime(t2);
 timestamp=dt_offset-magic;
 mesgTimestamp = timestamp;
}

/"timestamp_16"/{
 timestamp_16=$2;

 /* Calculate new timestamp based on delta */
 mesgTimestamp += and ( ( timestamp_16 - and ( mesgTimestamp , 0xFFFF ) ) , 0xFFFF);
 tt=strftime("%Y-%m-%dT%H:%M:%S",mesgTimestamp+magic)
 print "\"timestamp\": \""tt"\","
}

{
 print
}
' |jq