fix: parse ISO times taking local time into account

Add handling for the different timezones during parsing ISO dates as there's
a difference in date parsing and date-times parsing in the ECMA-script specs.
Fix the bit where even dates with given timezones were parsed as local time.

Related To #9941

src/util/DateUtils.ts

@@ -48,7 +48,7 @@ export class DateUtils {
     ): Date {
         let date =
             typeof mixedDate === "string"
-                ? DateUtils.parseDateAsISO(mixedDate)
+                ? DateUtils.parseDateAsISOLocal(mixedDate)
                 : mixedDate
 
         if (toUtc)
@@ -268,21 +268,37 @@ export class DateUtils {
     }
 
     /**
-     * Parse a date without the UTC-offset of the device
+     * Parse a ISO 8601 date(-time) string taking into account the local timezone (if none is given).
      *
-     * The problem here is with wrong timezone.
+     * The problem here is with wrong timezone and ECMA-Script being weird about when to use time zones and when not.
+     *
+     * **Caution:** The function expects an ISO8601 date(-time) string as input.
      *
      * For example:
      *
      * ``new Date('2021-04-28')`` will generate `2021-04-28T00:00:00.000Z`
      * in my timezone, which is not true for my timezone (GMT-0300). It should
      * be `2021-04-28T03:00:00.000Z` as `new Date(2021, 3, 28)` generates.
      */
-    private static parseDateAsISO(dateString: string): Date {
-        const date = new Date(dateString)
-        const offset = date.getTimezoneOffset() * 60 * 1000
-        const utc = date.getTime() + offset
-
-        return new Date(utc)
+    private static parseDateAsISOLocal(isoDateString: string): Date {
+        // From ECMA-262 specification:
+        // "The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of the String. [...]
+        //  When the UTC offset representation is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time."
+        // See https://tc39.es/ecma262/#sec-date-time-string-format for further information
+
+        const isDateTime = isoDateString.includes("T")
+        if (!isDateTime) {
+            // This lets the date parser now that the date is to be treat as local time.
+            isoDateString = `${isoDateString}T00:00:00`
+        }
+        const parsed = new Date(isoDateString)
+        console.log(
+            "isIsoDataTime",
+            isDateTime,
+            isoDateString,
+            parsed,
+            parsed.toISOString(),
+        )
+        return parsed
     }
 }