diff --git a/binder/requirements.txt b/binder/requirements.txt
index a0e20f3..1e47294 100644
--- a/binder/requirements.txt
+++ b/binder/requirements.txt
@@ -1,6 +1,6 @@
 # required packages for backstroke
 matplotlib>=3
 numpy<2.3  # up to date as of 1/2025. v2.3 requires py >3.10. seems OK otherwise
-pandas<2.3  # up to date as of 1/2025. v2.3 looks OK if no copy-on-write issues. v3 requires py >3.10 but seems OK otherwise
+pandas<3  # last changed 4/2026. v3 requires py >3.10 but seems OK otherwise
 requests>=2.32.2
 python-dotenv
diff --git a/simulator.py b/simulator.py
index 8826d53..0f24900 100755
--- a/simulator.py
+++ b/simulator.py
@@ -278,7 +278,7 @@ def portfolio_value(self, date=None, main_portfolio=True, at_close=True):
         Arguments
         ---------
 
-        date : `pandas.Timestamp` or `datetime.datetime`, optional
+        date : str or `pandas.Timestamp` or `datetime.datetime`, optional
             The date whose price data is used in the value calculation. Will
             cause an error if the date is not between the class' open and close
             dates or if it is not a market day. [default: self.today]
@@ -297,7 +297,7 @@ def portfolio_value(self, date=None, main_portfolio=True, at_close=True):
             raise ValueError("'at_close' must be a bool.")
 
         # ensure that the requested date is within the class instance's range
-        date = self.today if date is None else date
+        date = self.today if date is None else pd.Timestamp(date)
         if not self.open_date <= date <= self.end_date:
             raise ValueError("`date` must occur between `self.open_date` and "
                              "`self.end_date`")
@@ -332,12 +332,12 @@ def call_tiingo(self, tick, open_date,
         tick : str, required
             The ticker of the asset whose data will be downloaded.
 
-        open_date : `pandas.Timestamp` or `datetime.datetime`, required
+        open_date : str or `pandas.Timestamp` or `datetime.datetime`, required
             The earliest date of historical data to be downloaded. (Note that
             when this method is called upon initializing HistoricalSimulator,
             this argument is self.open_date, not self.start_date.)
 
-        end_date : `pandas.Timestamp` or `datetime.datetime`, optional
+        end_date : str or `pandas.Timestamp` or `datetime.datetime`, optional
             The final date of historical data to be downloaded. (When this
             method is called upon initializing HistoricalSimulator, this
             argument is self.end_date.) [default: pandas.Timestamp.today()]
@@ -345,8 +345,9 @@ def call_tiingo(self, tick, open_date,
         verbose : boolean, optional
             Whether or not to print the download's progress. [default: True]
         '''
-        open_date = open_date.strftime('%Y-%m-%d') # (e.g. '1998-07-13')
-        end_date = end_date.strftime('%Y-%m-%d')
+        open_date = pd.Timestamp(open_date).strftime('%Y-%m-%d')
+        end_date = pd.Timestamp(end_date).strftime('%Y-%m-%d')
+        # (e.g. '1998-07-13')
         if verbose:
             print(f"{tick} from {open_date} to {end_date}...")
 
@@ -383,7 +384,7 @@ def append_date(self, date, price_dict, verbose=False):
         Arguments
         ---------
 
-        date : `pandas.Timestamp` or `datetime.datetime`, required
+        date : str or `pandas.Timestamp` or `datetime.datetime`, required
             The new date to add to the simulation. Must be a weekday and must
             not be timezone-aware.