Modified data to have EOD time stamp attached. Still doesn't work. Output:

                     Open      High       Low       Close     Volume

Date
2000-01-03 17:30:00 101.3166 101.3166 98.3267 99.394493 8164300
2000-01-04 17:30:00 98.0917 98.4548 95.4328 95.507530 8089800
2000-01-05 17:30:00 95.6357 96.7249 93.7990 95.678391 12177900
2000-01-06 17:30:00 95.4221 96.7035 94.1407 94.140717 6227200
2000-01-07 17:30:00 95.8920 99.6081 95.7211 99.608055 8066500
... ... ... ... ... ...
2020-04-20 17:30:00 282.6100 286.7900 281.3500 281.589996 100109300
2020-04-21 17:30:00 276.7300 278.0400 272.0200 273.040009 126385700
2020-04-22 17:30:00 278.3500 281.0000 276.9100 279.100006 93524600
2020-04-23 17:30:00 280.4900 283.9400 278.7500 279.079987 104709700
2020-04-24 17:30:00 280.7300 283.7000 278.5000 282.970001 85063200

[5110 rows x 5 columns]
ROI: 0.00%

Is it possible that this is due to the date format of the pandas date? i.e. there is no time in my data source, vs in the examples on

https://www.backtrader.com/docu/timers/timers/

they all have times attached to their dates.

Having the same problem! Sharing whole code snippet below.

import datetime
import backtrader as bt

class BuyAndHold_More(bt.Strategy):
    params = dict(
        monthly_cash=1000.0,  # amount of cash to buy every month
    )

    def start(self):
        self.cash_start = self.broker.get_cash()
        self.val_start = 100.0

        # Add a timer which will be called on the 1st trading day of the month
        self.add_timer(
            bt.timer.SESSION_END,  # when it will be called
            monthdays=[1],  # called on the 1st day of the month
            monthcarry=True,  # called on the 2nd day if the 1st is holiday
        )

    def notify_timer(self, timer, when, *args, **kwargs):
        # Add the influx of monthly cash to the broker
        self.broker.add_cash(self.p.monthly_cash)
        print('Cash Added!')
        # buy available cash
        target_value = self.broker.get_value() + self.p.monthly_cash
        self.order_target_value(target=target_value)

    def stop(self):
        # calculate the actual returns
        self.roi = (self.broker.get_value() / self.cash_start) - 1.0
        print('ROI:        {:.2f}%'.format(self.roi))


if __name__ == '__main__':
    cerebro = bt.Cerebro()

   # Get a pandas dataframe

    class PandasData(bt.feeds.DataBase):
      #lines = ('adj_close')
      params = (
          ('datetime', 'Date'),
          ('open','Open'),
          ('high','High'),
          ('low','Low'),
          ('close','Close'),
          ('volume','Volume'),
          ('openinterest',None),
          #('adj_close','Adj Close'),
        )
        
    # import data
    print(df_new)
    adj_data = bt.feeds.PandasData(dataname=df_new)
    
    #add data to cerebro
    cerebro.adddata(adj_data)

    cerebro.addstrategy(BuyAndHold_More)

    # Execute
    cerebro.run()

Output when run. Note that my "Cash Added!" never prints, so it looks like the timer is never getting run.

            Open      High       Low       Close     Volume

Date
2000-01-03 101.3166 101.3166 98.3267 99.394493 8164300
2000-01-04 98.0917 98.4548 95.4328 95.507530 8089800
2000-01-05 95.6357 96.7249 93.7990 95.678391 12177900
2000-01-06 95.4221 96.7035 94.1407 94.140717 6227200
2000-01-07 95.8920 99.6081 95.7211 99.608055 8066500
... ... ... ... ... ...
2020-04-20 282.6100 286.7900 281.3500 281.589996 100109300
2020-04-21 276.7300 278.0400 272.0200 273.040009 126385700
2020-04-22 278.3500 281.0000 276.9100 279.100006 93524600
2020-04-23 280.4900 283.9400 278.7500 279.079987 104709700
2020-04-24 280.7300 283.7000 278.5000 282.970001 85063200

[5110 rows x 5 columns]
ROI: 0.00%