diff --git a/my/core/py37.py b/my/core/py37.py
new file mode 100644
index 0000000..6a52593
--- /dev/null
+++ b/my/core/py37.py
@@ -0,0 +1,122 @@
+# borrowed from /usr/lib/python3.7/datetime.py
+from datetime import datetime, timezone, timedelta
+
+def _parse_isoformat_date(dtstr):
+    # It is assumed that this function will only be called with a
+    # string of length exactly 10, and (though this is not used) ASCII-only
+    year = int(dtstr[0:4])
+    if dtstr[4] != '-':
+        raise ValueError('Invalid date separator: %s' % dtstr[4])
+
+    month = int(dtstr[5:7])
+
+    if dtstr[7] != '-':
+        raise ValueError('Invalid date separator')
+
+    day = int(dtstr[8:10])
+
+    return [year, month, day]
+
+
+def _parse_hh_mm_ss_ff(tstr):
+    # Parses things of the form HH[:MM[:SS[.fff[fff]]]]
+    len_str = len(tstr)
+
+    time_comps = [0, 0, 0, 0]
+    pos = 0
+    for comp in range(0, 3):
+        if (len_str - pos) < 2:
+            raise ValueError('Incomplete time component')
+
+        time_comps[comp] = int(tstr[pos:pos+2])
+
+        pos += 2
+        next_char = tstr[pos:pos+1]
+
+        if not next_char or comp >= 2:
+            break
+
+        if next_char != ':':
+            raise ValueError('Invalid time separator: %c' % next_char)
+
+        pos += 1
+
+    if pos < len_str:
+        if tstr[pos] != '.':
+            raise ValueError('Invalid microsecond component')
+        else:
+            pos += 1
+
+            len_remainder = len_str - pos
+            if len_remainder not in (3, 6):
+                raise ValueError('Invalid microsecond component')
+
+            time_comps[3] = int(tstr[pos:])
+            if len_remainder == 3:
+                time_comps[3] *= 1000
+
+    return time_comps
+
+
+def _parse_isoformat_time(tstr):
+    # Format supported is HH[:MM[:SS[.fff[fff]]]][+HH:MM[:SS[.ffffff]]]
+    len_str = len(tstr)
+    if len_str < 2:
+        raise ValueError('Isoformat time too short')
+
+    # This is equivalent to re.search('[+-]', tstr), but faster
+    tz_pos = (tstr.find('-') + 1 or tstr.find('+') + 1)
+    timestr = tstr[:tz_pos-1] if tz_pos > 0 else tstr
+
+    time_comps = _parse_hh_mm_ss_ff(timestr)
+
+    tzi = None
+    if tz_pos > 0:
+        tzstr = tstr[tz_pos:]
+
+        # Valid time zone strings are:
+        # HH:MM               len: 5
+        # HH:MM:SS            len: 8
+        # HH:MM:SS.ffffff     len: 15
+
+        if len(tzstr) not in (5, 8, 15):
+            raise ValueError('Malformed time zone string')
+
+        tz_comps = _parse_hh_mm_ss_ff(tzstr)
+        if all(x == 0 for x in tz_comps):
+            tzi = timezone.utc
+        else:
+            tzsign = -1 if tstr[tz_pos - 1] == '-' else 1
+
+            td = timedelta(hours=tz_comps[0], minutes=tz_comps[1],
+                           seconds=tz_comps[2], microseconds=tz_comps[3])
+
+            tzi = timezone(tzsign * td)
+
+    time_comps.append(tzi)
+
+    return time_comps
+
+def fromisoformat(date_string, cls=datetime):
+    """Construct a datetime from the output of datetime.isoformat()."""
+    if not isinstance(date_string, str):
+        raise TypeError('fromisoformat: argument must be str')
+
+    # Split this at the separator
+    dstr = date_string[0:10]
+    tstr = date_string[11:]
+
+    try:
+        date_components = _parse_isoformat_date(dstr)
+    except ValueError:
+        raise ValueError('Invalid isoformat string: %s' % date_string)
+
+    if tstr:
+        try:
+            time_components = _parse_isoformat_time(tstr)
+        except ValueError:
+            raise ValueError('Invalid isoformat string: %s' % date_string)
+    else:
+        time_components = [0, 0, 0, 0, None]
+
+    return cls(*(date_components + time_components))