Correct computation of date/times for most-used calendar intervals (#97)

* Correct computation of date/times for most-used calendar intervals

* Update setup.cfg

* Update pyproject.toml

---------

Co-authored-by: Karl Tarbet <ktarbet@users.noreply.github.com>

Author: perrymanmd

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "hec-dss-python"
-version = "0.1.23"
+version = "0.1.24"
 description = "Python wrapper for the HEC-DSS file database C library."
 authors = ["Hydrologic Engineering Center"]
 license = "MIT"

setup.cfg

@@ -1,6 +1,6 @@
 [metadata]
 name = hecdss
-version = 0.1.23
+version = 0.1.24
 author = Hydrologic Engineering Center
 author_email =hec.dss@usace.army.mil
 description = Python wrapper for the HEC-DSS file database C library.

src/hecdss/regular_timeseries.py

@@ -159,10 +159,55 @@ def _interval_to_times(self, new_interval):
         Args:
             new_interval (int): The new interval in seconds.
         """
+        is_leap = lambda y: y % 4 == 0 and y % 100 != 0 or y % 400 == 0
+        last_day = lambda y, m: 31 if m in (1,3,5,7,8,10,12) else 30 if m in (4,6,9,11) else 29 if is_leap(y) else 28
         if type(self.start_date) == datetime:
-            self.times = []
-            for i in range(len(self.values)):
-                self.times.append((self.start_date + (i * timedelta(seconds=new_interval))).replace(microsecond=0, tzinfo=ZoneInfo(self.time_zone_name) if self.time_zone_name else None))
+            tz = ZoneInfo(self.time_zone_name) if self.time_zone_name else None
+            first_time = self.start_date.replace(microsecond=0, tzinfo=tz)
+            count = len(self.values)
+            if new_interval <= 604800:
+                # --------------------- #
+                # non-calendar interval #
+                # --------------------- #
+                span = timedelta(seconds=new_interval)
+                self.times = [(first_time + i * span) for i in range(count)]
+            elif new_interval == 864000:
+                # ------------------ #
+                # Tri-Month interval #
+                # ------------------ #
+                raise ValueError("Tri-Month interval not currently supported")
+            elif new_interval == 1296000:
+                # ------------------- #
+                # Semi-Month interval #
+                # ------------------- #
+                raise ValueError("Semi-Month interval not currently supported")
+            elif new_interval == 2592000:
+                # ---------------- #
+                # 1-Month interval #
+                # ---------------- #
+                self.times = [first_time]
+                for i in range(1, count):
+                    y, m, d, h, n, s = self.times[-1].timetuple()[:6]
+                    if m == 12:
+                        y += 1
+                        m = 1
+                    else:
+                        m += 1
+                    d = min(last_day(y, m), first_time.day)
+                    self.times.append(datetime(y, m, d, h, n, s, tzinfo=tz))
+            elif new_interval == 31536000:
+                # --------------- #
+                # 1-Year interval #
+                # --------------- #
+                self.times = [first_time]
+                for i in range(1, count):
+                    y, m, d, h, n, s = self.times[-1].timetuple()[:6]
+                    y += 1
+                    d = min(last_day(y, m), first_time.day)
+                    self.times.append(datetime(y, m, d, h, n, s, tzinfo=tz))
+            else:
+                raise ValueError(f"Invalid interval seconds: {new_interval}")
+
 
     def _generate_times(self):
         """