Preliminary interval range add/subtract [doesn't work for all cases]

Author: rlskoeser

src/undate/converters/calendars/gregorian.py

@@ -40,7 +40,7 @@ def max_day(self, year: int, month: int) -> int:
             _, max_day = monthrange(year, month)
         else:
             # if year and month are unknown, return maximum possible
-            # TODO: should this return a ufloat?
+            # TODO: should this return an IntervalRange?
             max_day = 31
 
         return max_day

src/undate/date.py

@@ -49,6 +49,24 @@ def __init__(self, lower: int, upper: int):
             raise ValueError(f"Lower value ({lower}) must be less than upper ({upper})")
         super().__init__(portion.closed(lower, upper))
 
+    def __sub__(self, other):
+        # portion default subtraction is to remove from the set, but
+        # we want to decrease the values
+        if isinstance(other, int):
+            return IntegerRange(self.lower - other, self.upper - other)
+        elif isinstance(other, IntegerRange):
+            # subtract the range respective endpoints
+            return IntegerRange(self.lower - other.lower, self.upper - other.upper)
+
+    def __add__(self, other):
+        # portion default subtraction is to remove from the set, but
+        # we want to decrease the values
+        if isinstance(other, int):
+            return IntegerRange(self.lower + other, self.upper + other)
+        elif isinstance(other, IntegerRange):
+            # the new range is the smallest possible value to the highest
+            return IntegerRange(self.lower + other.lower, self.upper - other.upper)
+
 
 @dataclass
 class Udelta: