Batch shift the creation date of files on Mac OS

# Batch shift the creation date of files on Mac OS
# Source: https://superuser.com/a/894991/105418
# to shift files dates with a reference file/date (the old GoPro always reset its time to 2009..)

ref_file=GOPR3440.MP4
new_time_for_that_file=0327160015 #date's format: 27th March 2015, 16:00

ref_file_timestamp=`stat -f %B -t %s "$ref_file"`
new_time_timestamp=`date -j $new_time_for_that_file +%s`
time_diff=$[$new_time_timestamp - $ref_file_timestamp]

for f in *; do
    old=$(stat -f %B -t %s "$f")
    new=$(date -r $(($old + $time_diff)) '+%m/%d/%Y %H:%M:%S')
    SetFile -d "$new" -m "$new" "$f"
done