fix: resolve the blue Moon against the calendar month of UTC in moon module in @observerly/astrometry

src/moon.ts

@@ -1091,16 +1091,20 @@ export const isBlueMoon = (datetime: Date): boolean => {
   // then it is considered a Blue Moon. However, it is impossible to have a
   // Blue Moon on any other days except the 29th, 30th, or 31st of a given month,
   // as the synodic month is 29.530588853 days long.
-  const today = datetime.getDate()
+  //
+  // N.B. The calendar month is the month of UTC, and not the month of the host system: the day of
+  // the month of the host is not the day of the month of UTC, and so the same instant would be a
+  // Blue Moon on a host in one timezone and not on a host in another:
+  const today = datetime.getUTCDate()
 
   // Calculate the fraction of the current day, in milliseconds, of the synodic month:
   const f = (LUNAR_SYNODIC_MONTH - 29) * 24 * 60 * 60 * 1000
 
   const ms =
-    datetime.getHours() * 60 * 60 * 1000 +
-    datetime.getMinutes() * 60 * 1000 +
-    datetime.getSeconds() * 1000 +
-    datetime.getMilliseconds()
+    datetime.getUTCHours() * 60 * 60 * 1000 +
+    datetime.getUTCMinutes() * 60 * 1000 +
+    datetime.getUTCSeconds() * 1000 +
+    datetime.getUTCMilliseconds()
 
   // If the time on the current day is less than the fraction of the day of the synodic month,
   // then the Moon is not a Blue Moon or if the day is not the 29th, 30th, or 31st:

tests/moon.spec.ts

@@ -544,6 +544,29 @@ describe('isBlueMoon', () => {
     expect(isBlueMoon(datetime)).toBe(false)
   })
 
+  it('should not depend on the timezone of the host system', () => {
+    // The calendar month is the month of UTC, and so the same instant is a Blue Moon wherever the
+    // host is. The instants below fall on the last day of the month of UTC, but on the first day
+    // of the next month for a host to the east of it, and, for the one late in the day, on the
+    // penultimate day of the month for a host to the west of it:
+    const timezone = process.env.TZ
+
+    try {
+      // N.B. The instant on the 29th is compared against the fraction of a day by which the
+      // synodic month exceeds 29 days, e.g., ~12.7 hours, and so it exercises the time of day as
+      // well as the day of the month:
+      for (const iso of ['1999-03-31T23:49:00Z', '2023-08-31T01:35:00Z', '2004-08-29T18:00:00Z']) {
+        for (const TZ of ['UTC', 'Pacific/Auckland', 'Asia/Tokyo', 'America/Los_Angeles']) {
+          process.env.TZ = TZ
+
+          expect(isBlueMoon(new Date(iso))).toBe(true)
+        }
+      }
+    } finally {
+      process.env.TZ = timezone
+    }
+  })
+
   it('should return true for August 31st 2023', () => {
     // We know there is a second full moon in August 2023:
     const datetime = new Date('2023-08-31T01:35:00Z')