BUG: fix Period.asfreq conversion near datetime(1, 1, 1)

doc/source/whatsnew/v0.23.0.txt

@@ -714,6 +714,7 @@ Datetimelike
 - Bug in :class:`Timestamp` and :func:`to_datetime` where a string representing a barely out-of-bounds timestamp would be incorrectly rounded down instead of raising ``OutOfBoundsDatetime`` (:issue:`19382`)
 - Bug in :func:`Timestamp.floor` :func:`DatetimeIndex.floor` where time stamps far in the future and past were not rounded correctly (:issue:`19206`)
 - Bug in :func:`to_datetime` where passing an out-of-bounds datetime with ``errors='coerce'`` and ``utc=True`` would raise ``OutOfBoundsDatetime`` instead of parsing to ``NaT`` (:issue:`19612`)
+- Bug in :func:`Period.asfreq` where periods near ``datetime(1, 1, 1)`` could be converted incorrectly (:issue:`19643`)
 -
 
 Timezones

pandas/_libs/src/period_helper.c

@@ -138,7 +138,7 @@ PANDAS_INLINE npy_int64 transform_via_day(npy_int64 ordinal,
 }
 
 static npy_int64 DtoB_weekday(npy_int64 absdate) {
-    return (((absdate) / 7) * 5) + (absdate) % 7 - BDAY_OFFSET;
+    return floordiv(absdate, 7) * 5 + mod_compat(absdate, 7) - BDAY_OFFSET;
 }
 
 static npy_int64 DtoB(struct date_info *dinfo,
@@ -245,7 +245,8 @@ static npy_int64 asfreq_UpsampleWithinDay(npy_int64 ordinal,
 static npy_int64 asfreq_BtoDT(npy_int64 ordinal, asfreq_info *af_info) {
     ordinal += BDAY_OFFSET;
     ordinal =
-        (((ordinal - 1) / 5) * 7 + mod_compat(ordinal - 1, 5) + 1 - ORD_OFFSET);
+        (floordiv(ordinal - 1, 5) * 7 + mod_compat(ordinal - 1, 5) + 1 -
+         ORD_OFFSET);
 
     return upsample_daytime(ordinal, af_info);
 }

pandas/_libs/tslibs/period.pyx

@@ -154,12 +154,30 @@ cdef inline int get_freq_group(int freq) nogil:
     return (freq // 1000) * 1000
 
 
-@cython.cdivision
+@cython.cdivision(False)  # specifically _dont_ use cdvision GH#19643
 cdef int64_t get_period_ordinal(int year, int month, int day,
                                 int hour, int minute, int second,
                                 int microseconds, int picoseconds,
                                 int freq) nogil:
-    """generate an ordinal in period space"""
+    """
+    Generate an ordinal in period space
+
+    Parameters
+    ----------
+    year : int
+    month : int
+    day : int
+    hour : int
+    minute : int
+    second : int
+    microseconds : int
+    picoseconds : int
+    freq : int
+
+    Returns
+    -------
+    period_ordinal : int64_t
+    """
     cdef:
         int64_t absdays, unix_date, seconds, delta
         int64_t weeks
@@ -190,7 +208,7 @@ cdef int64_t get_period_ordinal(int year, int month, int day,
         if month >= fmonth:
             mdiff += 12
 
-        return (year - 1970) * 4 + (mdiff - 1) / 3
+        return (year - 1970) * 4 + (mdiff - 1) // 3
 
     elif freq == FR_MTH:
         return (year - 1970) * 12 + month - 1
@@ -202,14 +220,14 @@ cdef int64_t get_period_ordinal(int year, int month, int day,
         seconds = unix_date * 86400 + hour * 3600 + minute * 60 + second
 
         if freq == FR_MS:
-            return seconds * 1000 + microseconds / 1000
+            return seconds * 1000 + microseconds // 1000
 
         elif freq == FR_US:
             return seconds * 1000000 + microseconds
 
         elif freq == FR_NS:
             return (seconds * 1000000000 +
-                    microseconds * 1000 + picoseconds / 1000)
+                    microseconds * 1000 + picoseconds // 1000)
 
         else:
             return seconds
@@ -229,7 +247,7 @@ cdef int64_t get_period_ordinal(int year, int month, int day,
     elif freq == FR_BUS:
         # calculate the current week assuming sunday as last day of a week
         # Jan 1 0001 is a Monday, so subtract 1 to get to end-of-week
-        weeks = (unix_date + ORD_OFFSET - 1) / 7
+        weeks = (unix_date + ORD_OFFSET - 1) // 7
         # calculate the current weekday (in range 1 .. 7)
         delta = (unix_date + ORD_OFFSET - 1) % 7 + 1
         # return the number of business days in full weeks plus the business
@@ -241,7 +259,7 @@ cdef int64_t get_period_ordinal(int year, int month, int day,
 
     elif freq_group == FR_WK:
         day_adj = freq - FR_WK
-        return (unix_date + ORD_OFFSET - (1 + day_adj)) / 7 + 1 - WEEK_OFFSET
+        return (unix_date + ORD_OFFSET - (1 + day_adj)) // 7 + 1 - WEEK_OFFSET
 
     # raise ValueError
 
@@ -272,6 +290,15 @@ cdef int64_t get_python_ordinal(int64_t period_ordinal, int freq) nogil:
     This corresponds to the number of days since Jan., 1st, 1AD.
     When the instance has a frequency less than daily, the proleptic date
     is calculated for the last day of the period.
+
+    Parameters
+    ----------
+    period_ordinal : int64_t
+    freq : int
+
+    Returns
+    -------
+    absdate : int64_t number of days since datetime(1, 1, 1)
     """
     cdef:
         asfreq_info af_info
@@ -290,6 +317,20 @@ cdef int dInfoCalc_SetFromAbsDateTime(date_info *dinfo,
     """
     Set the instance's value using the given date and time.
     Assumes GREGORIAN_CALENDAR.
+
+    Parameters
+    ----------
+    dinfo : date_info*
+    absdate : int64_t (as returned from get_python_ordinal or absdate_from_ymd)
+    abstime : double
+
+    Returns
+    -------
+    code : int (always 0)
+
+    Notes
+    -----
+    Updates dinfo inplace
     """
     # Bounds check
     # The calling function is responsible for ensuring that
@@ -307,6 +348,19 @@ cdef int dInfoCalc_SetFromAbsDate(date_info *dinfo, int64_t absdate) nogil:
     """
     Sets the date part of the date_info struct
     Assumes GREGORIAN_CALENDAR
+
+    Parameters
+    ----------
+    dinfo : date_info*
+    unix_date : int64_t
+
+    Returns
+    -------
+    code : int (always 0)
+
+    Notes
+    -----
+    Updates dinfo inplace
     """
     cdef:
         pandas_datetimestruct dts
@@ -322,6 +376,19 @@ cdef int dInfoCalc_SetFromAbsDate(date_info *dinfo, int64_t absdate) nogil:
 cdef int dInfoCalc_SetFromAbsTime(date_info *dinfo, double abstime) nogil:
     """
     Sets the time part of the DateTime object.
+
+    Parameters
+    ----------
+    dinfo : date_info*
+    abstime : double
+
+    Returns
+    -------
+    code : int (always 0)
+
+    Notes
+    -----
+    Updates dinfo inplace
     """
     cdef:
         int inttime
@@ -370,7 +437,18 @@ cdef int64_t absdate_from_ymd(int year, int month, int day) nogil:
     Find the absdate (days elapsed since datetime(1, 1, 1)
     for the given year/month/day.
     Assumes GREGORIAN_CALENDAR
+
+    Parameters
+    ----------
+    year : int
+    month : int
+    day : int
+
+    Returns
+    -------
+    absdate : int days elapsed since datetime(1, 1, 1)
     """
+
     # /* Calculate the absolute date
     cdef:
         pandas_datetimestruct dts
@@ -385,6 +463,24 @@ cdef int64_t absdate_from_ymd(int year, int month, int day) nogil:
 
 
 cdef int get_yq(int64_t ordinal, int freq, int *quarter, int *year):
+    """
+    Find the year and quarter of a Period with the given ordinal and frequency
+
+    Parameters
+    ----------
+    ordinal : int64_t
+    freq : int
+    quarter : *int
+    year : *int
+
+    Returns
+    -------
+    qtr_freq : int describing the implied quarterly frequency
+
+    Notes
+    -----
+    Sets quarter and year inplace
+    """
     cdef:
         asfreq_info af_info
         int qtr_freq