python pagetimes.py infile.csv outfile.csv
from sys import argv
from csv import DictReader, DictWriter

times = {}

with open(argv[1]) as inf, open(argv[2], 'w', newline='') as outf:
    reader = DictReader(inf)
    writer = DictWriter(outf, fieldnames=reader.fieldnames + ['seconds_on_page'])
    writer.writeheader()
    for row in reader:
        pcode = row['participant_code']
        # to learn what Epoch time is, search on Google.
        time = int(row.get('epoch_time_completed') or row.get('epoch_time'))
        last_time = times.get(pcode)
        if last_time:
            delta = time - last_time
        else:
            delta = ''
        row['seconds_on_page'] = delta
        writer.writerow(row)
        times[pcode] = time