Fix: Date parsing timezone bug in coerceToDate utility

src/date.ts

@@ -6,15 +6,18 @@
  * - 13-digit timestamps (Unix milliseconds) - used directly
  * - String representations of timestamps
  * - ISO date strings (e.g., "2025-06-01T12:30:00Z")
- * - Simple date strings (e.g., "2025-06-01") - time defaults to 00:00:00
+ * - Simple date strings (e.g., "2025-06-01") - time defaults to 00:00:00 UTC
  * - Date strings with space-separated time (e.g., "2025-06-01 12:30:00")
  *
  * Note: This function follows JavaScript Date constructor behavior for date parsing.
  * Some invalid dates like "2025-02-30" auto-correct to valid dates (becomes "2025-03-02"),
  * while malformed strings like "2025-13-01" or "2025-06-01T25:00:00" return null.
  *
+ * For simple date strings without time components, the function uses UTC methods to ensure
+ * consistent behavior across all timezones, avoiding timezone shift bugs in date comparisons.
+ *
  * @example
- * coerceToDate('2025-06-01') // Returns Date object set to 2025-06-01 00:00:00
+ * coerceToDate('2025-06-01') // Returns Date object set to 2025-06-01 00:00:00 UTC
  * coerceToDate('2025-06-01T12:30:00Z') // Returns Date object with specified time
  * coerceToDate(1748834578) // Returns Date object (10-digit timestamp in seconds)
  * coerceToDate(1748834578000) // Returns Date object (13-digit timestamp in milliseconds)
@@ -53,12 +56,12 @@ export const coerceToDate = (
     // Return null for invalid dates
     if (Number.isNaN(dateObj.getTime())) return null;
 
-    // If no time component is specified, set time to 00:00:00
-    // this is because by default JS will set the time to midnight UTC for that date
+    // If no time component is specified, set time to 00:00:00 UTC
+    // Use setUTCHours to avoid timezone shifts
     const hasTimeComponent =
       /T\d{2}:\d{2}(:\d{2})?/.test(dateInput) || /\d{2}:\d{2}/.test(dateInput);
     if (!hasTimeComponent) {
-      dateObj.setHours(0, 0, 0, 0);
+      dateObj.setUTCHours(0, 0, 0, 0);
     }
 
     return dateObj;

test/date.test.ts

@@ -72,14 +72,14 @@ describe('coerceToDate', () => {
   });
 
   describe('simple date string inputs', () => {
-    it('should handle simple date strings and set time to 00:00:00', () => {
+    it('should handle simple date strings and set time to 00:00:00 UTC', () => {
       const dateString = '2025-06-01';
       const result = coerceToDate(dateString);
       expect(result).toBeInstanceOf(Date);
-      expect(result?.getHours()).toBe(0);
-      expect(result?.getMinutes()).toBe(0);
-      expect(result?.getSeconds()).toBe(0);
-      expect(result?.getMilliseconds()).toBe(0);
+      expect(result?.getUTCHours()).toBe(0);
+      expect(result?.getUTCMinutes()).toBe(0);
+      expect(result?.getUTCSeconds()).toBe(0);
+      expect(result?.getUTCMilliseconds()).toBe(0);
     });
 
     it('should handle date strings with slashes', () => {