Normative: Allow ECMAScript code to specify UTC offset time zones

Fixes #683

spec/datetimeformat.html

@@ -94,9 +94,13 @@ <h1>
           1. Set _timeZone_ to DefaultTimeZone().
         1. Else,
           1. Set _timeZone_ to ? ToString(_timeZone_).
-          1. If the result of IsValidTimeZoneName(_timeZone_) is *false*, then
-            1. Throw a *RangeError* exception.
+        1. If IsTimeZoneOffsetString(_timeZone_) is *true*, then
+          1. Let _offsetNanoseconds_ be ParseTimeZoneOffsetString(_timeZone_).
+          1. Set _timeZone_ to FormatTimeZoneOffsetString(_offsetNanoseconds_).
+        1. Else if IsValidTimeZoneName(_timeZone_) is *true*, then
           1. Set _timeZone_ to CanonicalizeTimeZoneName(_timeZone_).
+        1. Else,
+          1. Throw a *RangeError* exception.
         1. Set _dateTimeFormat_.[[TimeZone]] to _timeZone_.
         1. Let _formatOptions_ be a new Record.
         1. Set _formatOptions_.[[hourCycle]] to _hc_.
@@ -164,6 +168,31 @@ <h1>
         1. Return _dateTimeFormat_.
       </emu-alg>
     </emu-clause>
+
+    <emu-clause id="sec-formattimezoneoffsetstring" aoid="FormatTimeZoneOffsetString">
+      <h1>FormatTimeZoneOffsetString ( _offsetNanoseconds_ )</h1>
+      <emu-alg>
+        1. Assert: _offsetNanoseconds_ is an integer.
+        1. If _offsetNanoseconds_ &ge; 0, let _sign_ be the code unit 0x002B (PLUS SIGN); otherwise, let _sign_ be the code unit 0x002D (HYPHEN-MINUS).
+        1. Set _offsetNanoseconds_ to abs(_offsetNanoseconds_).
+        1. Let _nanoseconds_ be _offsetNanoseconds_ modulo 10<sup>9</sup>.
+        1. Let _seconds_ be floor(_offsetNanoseconds_ / 10<sup>9</sup>) modulo 60.
+        1. Let _minutes_ be floor(_offsetNanoseconds_ / (6 &times; 10<sup>10</sup>)) modulo 60.
+        1. Let _hours_ be floor(_offsetNanoseconds_ / (3.6 &times; 10<sup>12</sup>)).
+        1. Let _h_ be ToZeroPaddedDecimalString(_hours_, 2).
+        1. Let _m_ be ToZeroPaddedDecimalString(_minutes_, 2).
+        1. Let _s_ be ToZeroPaddedDecimalString(_seconds_, 2).
+        1. If _nanoseconds_ &ne; 0, then
+          1. Let _fraction_ be ToZeroPaddedDecimalString(_nanoseconds_, 9).
+          1. Set _fraction_ to the longest possible substring of _fraction_ starting at position 0 and not ending with the code unit 0x0030 (DIGIT ZERO).
+          1. Let _post_ be the string-concatenation of the code unit 0x003A (COLON), _s_, the code unit 0x002E (FULL STOP), and _fraction_.
+        1. Else if seconds &ne; 0, then
+          1. Let _post_ be the string-concatenation of the code unit 0x003A (COLON) and _s_.
+        1. Else,
+          1. Let _post_ be the empty String.
+        1. Return the string-concatenation of _sign_, _h_, the code unit 0x003A (COLON), _m_, and _post_.
+      </emu-alg>
+    </emu-clause>
   </emu-clause>
 
   <emu-clause id="sec-properties-of-intl-datetimeformat-constructor">