From 9e0cb4f71b4d4fa71e88940f6d46b428603db508 Mon Sep 17 00:00:00 2001
From: anishnya <anishnya@umich.edu>
Date: Wed, 26 May 2021 03:44:04 -0400
Subject: [PATCH] Added new Excel Date Feature.

---
 arrow/arrow.py      | 37 ++++++++++++++++++++++++++++++++++++
 docs/index.rst      | 12 ++++++++++++
 tests/test_arrow.py | 46 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 95 insertions(+)

diff --git a/arrow/arrow.py b/arrow/arrow.py
index d01fa894..6421d681 100644
--- a/arrow/arrow.py
+++ b/arrow/arrow.py
@@ -32,6 +32,7 @@
 from dateutil import tz as dateutil_tz
 from dateutil.relativedelta import relativedelta
 
+import arrow
 from arrow import formatter, locales, parser, util
 from arrow.constants import DEFAULT_LOCALE, DEHUMANIZE_LOCALES
 from arrow.locales import TimeFrameLiteral
@@ -1787,6 +1788,42 @@ def _get_tzinfo(tz_expr: Optional[TZ_EXPR]) -> dt_tzinfo:
             except parser.ParserError:
                 raise ValueError(f"{tz_expr!r} not recognized as a timezone.")
 
+    @classmethod
+    def excel_date(
+        cls, delta: Union[int, float], default_windows_date: bool = True
+    ) -> "Arrow":
+        """Returns a new :class:`Arrow <arrow.arrow.Arrow>` object, that represents
+        the date of an Excel Serial formatted date.
+
+        :param delta: a ``int`` or ``float`` representing an Excel Serial Date.
+        :param default_windows_date: (optional) a ``bool`` specifying whether a user
+        wants to use the Windows or macOS date system.  Defaults to 'True' for Windows
+        date system.
+
+        Usage::
+
+                >>> arw = Arrow.excel_date(34519)
+                >>> arw
+                <Arrow [1994-07-04T00:00:00+00:00]>
+                >>> arw = Arrow.excel_date(34519, default_windows_date = False)
+                >>> arw
+                <Arrow [1998-07-05T00:00:00+00:00]>
+
+        """
+
+        if default_windows_date:
+            # Need to have this clause as Excel incorrectly considers 1900 a leap year
+            if delta < 60:
+                start_date = arrow.get("1899-12-31")
+            else:
+                start_date = arrow.get("1899-12-30")
+        else:
+            start_date = arrow.get("1904-01-01")
+
+        shifted_time = start_date.shift(days=delta)
+
+        return shifted_time
+
     @classmethod
     def _get_datetime(
         cls, expr: Union["Arrow", dt_datetime, int, float, str]
diff --git a/docs/index.rst b/docs/index.rst
index 43895b04..e1631523 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -82,6 +82,18 @@ Arrow objects can be instantiated directly too, with the same arguments as a dat
     >>> arrow.Arrow(2013, 5, 5)
     <Arrow [2013-05-05T00:00:00+00:00]>
 
+Arrow objects can be instantiated using the Excel Serial Date format. Both Windows and macOS date systems supported:
+
+    .. code-block:: python
+
+        >>> arw = Arrow.excel_date(34519)
+        >>> arw
+        <Arrow [1994-07-04T00:00:00+00:00]>
+
+        >>> arw = Arrow.excel_date(34519, default_windows_date = False)
+        >>> arw
+        <Arrow [1998-07-05T00:00:00+00:00]>
+
 Properties
 ~~~~~~~~~~
 
diff --git a/tests/test_arrow.py b/tests/test_arrow.py
index bc72bf31..dc05b557 100644
--- a/tests/test_arrow.py
+++ b/tests/test_arrow.py
@@ -2867,6 +2867,52 @@ def test_value_error_exception(self):
             target.span("week", week_start=55)
 
 
+class TestExcelDate:
+    def test_windows_date_leap_year_edge_case(self):
+        start = arrow.Arrow.fromdatetime(datetime(1900, 2, 28))
+        test = arrow.Arrow.excel_date(59)
+        assert start == test
+
+    def test_windows_date_leap_year_edge_case_after(self):
+        # Extra day loops back to be 02/28/1900
+        start = arrow.Arrow.fromdatetime(datetime(1900, 2, 28))
+        test = arrow.Arrow.excel_date(60)
+        assert start == test
+
+    def test_windows_date_begin(self):
+        # Testing in Excel shows 1/1/1900 is serial number 1,
+        start = arrow.Arrow.fromdatetime(datetime(1900, 1, 1))
+        test = arrow.Arrow.excel_date(1)
+        assert start == test
+
+    def test_windows_date_large(self):
+        start = arrow.Arrow.fromdatetime(datetime(1994, 7, 4))
+        test = arrow.Arrow.excel_date(34519)
+        assert start == test
+
+    def test_mac_date_leap_year_edge_case(self):
+        start = arrow.Arrow.fromdatetime(datetime(1904, 2, 29))
+        test = arrow.Arrow.excel_date(59, default_windows_date=False)
+        assert start == test
+
+    def test_mac_date_leap_year_edge_case_after(self):
+        # Should be no lopp back as no edge case with macOS date system
+        start = arrow.Arrow.fromdatetime(datetime(1904, 3, 1))
+        test = arrow.Arrow.excel_date(60, default_windows_date=False)
+        assert start == test
+
+    def test_mac_date_begin(self):
+        # Testing in Excel shows 1/1/1904 is serial number 0,
+        start = arrow.Arrow.fromdatetime(datetime(1904, 1, 1))
+        test = arrow.Arrow.excel_date(0, default_windows_date=False)
+        assert start == test
+
+    def test_mac_date_large(self):
+        start = arrow.Arrow.fromdatetime(datetime(1998, 7, 5))
+        test = arrow.Arrow.excel_date(34519, default_windows_date=False)
+        assert start == test
+
+
 class TestArrowUtil:
     def test_get_datetime(self):