update_timezones.py

# update_timezones.py
# Aleksandr Pasechnik
#
# Goes through the Day One jounal and sets the Time Zone of each entry that
# doesn't already have one to the value of the *timezone* variable. Makes a
# backup copy of each entry it modified by adding a '.tzbak' to the filename.
# Ignores any entry that already has a '.tzbak' version.
# 
# NOTE: base_dir may need to be adjusted to the correct Journal_dayone location
# NOTE: It is probably a good idea to have a full journal backup just in case
# something goes wrong

import os
import plistlib
import shutil

timezone = 'America/New_York'
base_dir = '~/Library/Mobile Documents/5U8NS4GX82~com~dayoneapp~dayone/Documents/Journal_dayone/'

base_dir = os.path.expanduser(base_dir)
entries_dir = os.path.join(base_dir,'entries')

files = os.listdir(entries_dir)
files[:] = [file for file in files if file.endswith('.doentry')]

for file in files:
	filename = os.path.join(entries_dir,file)
	entry = plistlib.readPlist(filename)
	if 'Time Zone' not in entry.keys():
		print 'Found an entry without a time zone:', entry['UUID']
		entry['Time Zone'] = timezone
		backupfilename = filename+'.tzbak'
		if not os.path.exists(backupfilename):
			print '	Backing up %s to %s'%(filename,backupfilename)
			shutil.move(filename,backupfilename)
			print '	Writing new entry at %s'%(filename)
			plistlib.writePlist(entry,filename)
		else:
			print '	Not doing anything because backup already exists at %s'%backupfilename
		print ''

print 'Done.'

In doing some investigating into timezones in old Day One entires recently, I found that the official hashtag convert script adds the current timezone to old entries with hashtags along with native tags. This means that you may end up with entries with the wrong timezone. For example, many of my old entries were written in eastern time, but since I live in central time now, my old entries with hashtags got tagged with central time.

This isn't an issue with your script (in fact, @bdolman says it's a bug on their end), but since you're interested in timezones in Day One, figured I'd let you know.