fix: helpers.subtract_years() should add +1 years when the month is december (12)

Author: chilango74

README.md

@@ -61,7 +61,7 @@ The package is supplied with **free** «end of day» historical stock markets da
 
 ## Getting started
 
-### 1. Compare several assets from different stock markets. Get the USD-adjusted perfomance
+### 1. Compare several assets from different stock markets. Get USD-adjusted performance
 
 ```python
 import okama as ok

main.py

@@ -1,3 +1,4 @@
 import okama as ok
 
-print(ok.symbols_in_namespace('CBR').info)
+x = ok.AssetList(['SXR8.XETR', 'SXRG.XETR', 'XRS2.XETR'])
+print(x.get_cagr(5))

okama/assets.py

@@ -117,6 +117,7 @@ def __init__(self,
             self.first_date: pd.Timestamp = max(self.first_date, pd.to_datetime(first_date))
         self.ror = self.ror[self.first_date:]
         if last_date:
+            # TODO: self.assets_last_dates should be less or equal to self.last_date
             self.last_date: pd.Timestamp = min(self.last_date, pd.to_datetime(last_date))
         self.ror: pd.DataFrame = self.ror[self.first_date: self.last_date]
         self.period_length: float = round((self.last_date - self.first_date) / np.timedelta64(365, 'D'), ndigits=1)

okama/helpers.py

@@ -425,12 +425,12 @@ def subtract_years(dt: pd.Timestamp, years: int) -> pd.Timestamp:
         """
         if isinstance(years, int):
             if dt.month == 12:
-                dt = dt.replace(year=dt.year - years, month=1)  # for December
+                new_dt = dt.replace(year=dt.year - years + 1, month=1)  # for December
             else:
-                dt = dt.replace(year=dt.year - years, month=dt.month + 1)
+                new_dt = dt.replace(year=dt.year - years, month=dt.month + 1)
         else:
             raise TypeError('The period should be integer')
-        return dt
+        return new_dt
 
 
 class Index: