diff --git a/fincal/fincal.py b/fincal/fincal.py
index 3f5450a..3751991 100644
--- a/fincal/fincal.py
+++ b/fincal/fincal.py
@@ -476,7 +476,7 @@ class TimeSeries(TimeSeriesCore):
             ValueError: If frequency string is outside valid values
 
         Also see:
-        --------
+        ---------
             TimeSeries.calculate_rolling_returns()
         """
 
@@ -606,12 +606,28 @@ class TimeSeries(TimeSeriesCore):
 
         return output_ts
 
-    def sync(self, other: TimeSeries, fill_method: Literal["ffill", "bfill"] = "ffill"):
+    def sync(self, other: TimeSeries, fill_method: Literal["ffill", "bfill"] = "ffill") -> TimeSeries:
         """Synchronize two TimeSeries objects
 
         This will ensure that both time series have the same frequency and same set of dates.
         The frequency will be set to the higher of the two objects.
         Dates will be taken from the class on which the method is called.
+
+        Parameters:
+        -----------
+        other: TimeSeries
+            Another object of TimeSeries class whose dates need to be syncronized
+
+        fill_method: ffill | bfill, default ffill
+            Method to use to fill missing values in time series when syncronizing
+
+        Returns:
+        --------
+            Returns another object of TimeSeries class
+
+        Raises:
+        --------
+            Raises TypeError if the other object is not of TimeSeries class
         """
 
         if not isinstance(other, TimeSeries):
@@ -622,9 +638,9 @@ class TimeSeries(TimeSeriesCore):
         if self.frequency.days > other.frequency.days:
             self = self.expand(to_frequency=other.frequency.symbol, method=fill_method)
 
-        new_other = {}
+        new_other: dict = {}
+        closest = "previous" if fill_method == "ffill" else "next"
         for dt in self.dates:
-            closest = "previous" if fill_method == "ffill" else "next"
             if dt in other:
                 new_other[dt] = other[dt][1]
             else:
diff --git a/fincal/utils.py b/fincal/utils.py
index 45de083..2c48276 100644
--- a/fincal/utils.py
+++ b/fincal/utils.py
@@ -13,9 +13,29 @@ class FincalOptions:
     get_closest: str = "exact"
 
 
-def _parse_date(date: str, date_format: str = None):
-    """Parses date and handles errors"""
-    # print(date, date_format)
+def _parse_date(date: str, date_format: str = None) -> datetime.datetime:
+    """Parses date and handles errors
+
+    Parameters:
+    -----------
+    date: str | datetime.date
+        The date to be parsed.
+        If the date passed is already a datetime object, it will return it unprocessed.
+
+    date_format: str, default None
+        The format of the date string in datetime.strftime friendly format.
+        If format is None, format in FincalOptions.date_format will be used.
+
+    Returns:
+    --------
+        Returns a datetime.datetime object.
+
+    Raises:
+    -------
+        TypeError: If the is not a date-like string
+        ValueError: If the date could not be parsed with the given format
+    """
+
     if isinstance(date, (datetime.datetime, datetime.date)):
         return datetime.datetime.fromordinal(date.toordinal())
 
@@ -37,18 +57,53 @@ def _preprocess_timeseries(
     | Mapping[str | datetime.datetime, float],
     date_format: str,
 ) -> List[Tuple[datetime.datetime, float]]:
-    """Converts any type of list to the correct type"""
+    """Converts any type of list to the TimeSeries friendly format.
+        This function is internally called by the __init__ function of the TimeSeriesCore class
+
+        The TimeSeries class can internally process a list of Tuples.
+        However, users have the option of passing a variety of types.
+        This function preprocesses the data and converts it into the relevant format.
+
+        If the data is a dictionary, it will be converted using .items() iteration.
+        If the data is not a dictionary or a list, it will raise an error.
+        If the data is of list type:
+            * If the first item is also of list type, it will be parsed as a list of lists
+            * If the first item is a dictionary with one key, then key will be parsed as date
+            * If the first item is a dictionary with two keys, then first key will be date and second will be value
+            * If the first element is of another type, it will raise an error
+
+        The final return value is sorted by date
+
+    Parameters:
+    -----------
+    Data:
+        The data for the time series. Can be a dictionary, a list of tuples, or a list of dictionaries.
+
+    date_format: str
+        The format of the date in strftime friendly format.
+
+    Returns:
+    -----------
+        Returns a list of Tuples where the first element of each tuple is of datetime.datetime class
+        and the second element is of float class
+
+    Raises:
+    --------
+    TypeError: If the data is not in a format which can be parsed.
+    """
 
     if isinstance(data, Mapping):
         current_data: List[tuple] = [(k, v) for k, v in data.items()]
         return _preprocess_timeseries(current_data, date_format)
 
+    # If data is not a dictionary or list, it cannot be parsed
     if not isinstance(data, Sequence):
         raise TypeError("Could not parse the data")
 
     if isinstance(data[0], Sequence):
         return sorted([(_parse_date(i, date_format), j) for i, j in data])
 
+    # If first element is not a dictionary or tuple, it cannot be parsed
     if not isinstance(data[0], Mapping):
         raise TypeError("Could not parse the data")