code: def get_start_dt(
      end_dt, bar_count, data_frequency, include_first=True
  ):
      """
      The start date based on specified end date and data frequency.
  
      Parameters
      ----------
      end_dt: datetime
      bar_count: int
      data_frequency: str
      include_first
  
      Returns
      -------
      datetime
  
      """
      periods = bar_count
      if periods > 1:
          delta = get_delta(periods, data_frequency)
          start_dt = end_dt - delta
  
          if not include_first:
              start_dt += get_delta(1, data_frequency)
      else:
          start_dt = end_dt
  
      return start_dt
  
file_path: catalyst/catalyst/exchange/utils/datetime_utils.py
imports: import calendar
  import math
  import re
  from datetime import datetime, timedelta, date
  import pandas
  import pytz
  from catalyst.exchange.exchange_errors import InvalidHistoryFrequencyError, InvalidHistoryFrequencyAlias
label_desc: Use image node_id 6 for calling a global function with example usage: get_start_dt(end_dt, bar_count, data_frequency, include_first) and returns: start_dt