Script to convert datetimes in an icalendar ics file to local time

Gemfile

source 'https://rubygems.org'

gem 'icalendar'
gem 'activesupport'

# gem 'pry'

ical_timezone_fixer.rb

# This (very quick&dirty) script parses an ics file and converts all datetimes to the given local timezone.
# Practical use-case is to prepare an ics file exported from Google Calendar (with a mix
# of UTC and local times) and prepare it for importing it to Nextcloud Calendar.

# configure here
input_filename = 'google_calendar.ics'
output_filename = 'google_calendar_converted.ics'
local_tzid = 'Europe/Berlin'

require 'bundler'
Bundler.require

cal_file = File.open(input_filename)

# Parser returns an array of calendars because a single file can have multiple calendars.
cals = Icalendar::Calendar.parse(cal_file)
cal = cals.first

# binding.pry

local_time_zone = ActiveSupport::TimeZone.new(local_tzid)

def convert_ical_datetime_to_timezone(datetime, timezone)
  Icalendar::Values::DateTime.new(datetime.in_time_zone(timezone), {'tzid' => timezone.tzinfo.name})
end

def need_to_convert_timezone?(datetime, local_time_zone)
  datetime.is_a?(Icalendar::Values::DateTime) && datetime.time_zone.name != local_time_zone.tzinfo.name
end

potential_date_time_attrs = %i[dtstart dtend dtstamp created last_modified]

cal.events.each do |event|
  potential_date_time_attrs.each do |date_time_attr|
    if need_to_convert_timezone?(event.send(date_time_attr), local_time_zone)
      event.send("#{date_time_attr}=".to_sym, convert_ical_datetime_to_timezone(event.send(date_time_attr), local_time_zone))
    end
  end
end

File.write(output_filename, cal.to_ical)