gistfile1.rb

require 'TZInfo'

class TimeZoneSearch
  def initialize(zones_to_search)
    @zones_to_search = zones_to_search
  end

  def zones_with_local_hour(hour_to_match, utc_time = Time.now.utc.to_datetime)
    zone_matches = []
    utc_minute = utc_time.minute

    # Round to the closest 15 minute mark.
    minute_to_check = utc_minute - (utc_minute % 15)
    
    # UTC time to the closest 15 minute mark excluding seconds.
    utc_time_to_check = Time.utc(utc_time.year, utc_time.month, utc_time.day, utc_time.hour, minute_to_check, 0)

    @zones_to_search.each do |zone_name|
      local_zone = time_zone(zone_name)
      local_time = local_zone.utc_to_local(utc_time_to_check).to_datetime

      # Check for 0 minute because offset minutes are multiples of 15.
      if local_time.minute == 0 && local_time.hour == hour_to_match
        zone_matches << local_zone.identifier
      end
    end

    zone_matches
  end

  def time_zone(name)
    TZInfo::Timezone.get(name)
  end
end