diff --git a/index.js b/index.js
index 5fbb3ad..74a3ec8 100644
--- a/index.js
+++ b/index.js
@@ -1,6 +1,6 @@
 "use strict";
 
-function fromUTCToTimeZone(date, timeZone) {
+function toTimeZone(date, timeZone) {
     // ISO string or existing date object
     date = new Date(date);
     var options = {
@@ -15,12 +15,7 @@ function fromUTCToTimeZone(date, timeZone) {
         fractionalSecondDigits: 3,
     };
 
-    var tzOptions = Object.assign(
-        {
-            timeZoneName: "long",
-        },
-        options
-    );
+    var tzOptions = Object.assign({ timeZoneName: "long" }, options);
 
     // Every country uses the same year and months, right?
     var formater = new Intl.DateTimeFormat("default", tzOptions);
@@ -61,6 +56,11 @@ function fromUTCToTimeZone(date, timeZone) {
     return whole;
 }
 
+function toTimeZoneISOString(date, timeZone) {
+    var whole = toTimeZone(date, timeZone);
+    return toOffsetISOString(whole);
+}
+
 function _toOffsetISOString() {
     return toOffsetISOString(this);
 }
@@ -109,18 +109,63 @@ function toOffsetISOString(d) {
     );
 }
 
-function fromZonedToUTC(dt, tz) {}
+function toUTC(dt, tz) {
+    if ("string" === typeof dt) {
+        // Either of these formats should work:
+        // 2021-03-14 01:15:59
+        // 2021-03-14T01:15:59Z
+        dt = dt
+            .replace("T", " ")
+            .replace("Z", "")
+            .replace(" ", "T")
+            .replace(/$/, "Z");
+    }
+    var utcDate = new Date(dt);
+    var tzD2 = toTimeZone(utcDate, tz);
+    var offset = tzD2.offset;
+    tzD2.offset = "";
+
+    var deltaDate = new Date(utcDate);
+    deltaDate.setUTCMinutes(deltaDate.getUTCMinutes() - offset);
+    var tzD3 = toTimeZone(deltaDate, tz);
+
+    if (
+        tzD3.hour === utcDate.getUTCHours() &&
+        tzD3.minute === utcDate.getUTCMinutes()
+    ) {
+        return tzD3;
+    }
+
+    var diff = tzD3.offset - offset;
+    var h = Math.floor(Math.abs(diff) / 60);
+    var m = Math.abs(diff) % 60;
+    var sign = Math.abs(diff) / diff;
+    tzD3.hour -= h * sign;
+    tzD3.minute -= m * sign;
+
+    return tzD3;
+}
+
+function toUTCISOString(date, timeZone) {
+    var whole = toUTC(date, timeZone);
+    return toOffsetISOString(whole);
+}
 
 module.exports = {
     // bespoke date =>
     // 2021-11-07T3:15:59-0500
     toOffsetISOString: toOffsetISOString,
+
     // -240 => -0400
     formatOffset: formatOffset,
+
     // [ "2021-11-07T08:15:59Z", "America/New_York" ]
     // => "2021-11-07T03:15:59-0500" // 2021-11-07 03:15:59
-    fromUTCToTimeZone: fromUTCToTimeZone,
+    toTimeZone: toTimeZone,
+    toTimeZoneISOString: toTimeZoneISOString,
+
     // [ "2021-11-07 03:15:59", "America/New_York" ]
     // => "2021-11-07T03:15:59-0500" // 2021-11-07T08:15:59Z
-    fromZonedToUTC: fromZonedToUTC,
+    toUTC: toUTC,
+    toUTCISOString: toUTCISOString,
 };
diff --git a/test.js b/test.js
index 7afce5f..42bc5fb 100644
--- a/test.js
+++ b/test.js
@@ -2,6 +2,29 @@
 
 var TZ = require("./");
 
+function testUtcToTz(t) {
+    var result = TZ.toTimeZone.apply(TZ, t.inputs).toISOString();
+    if (t.result !== result) {
+        throw new Error(
+            `Invalid UTC to TZ conversion for ${t.desc}:\n` +
+                `\tExpected: ${t.result}\n` +
+                `\tActual: ${result}\n`
+        );
+    }
+}
+
+function testTzToUtc(t) {
+    var result = TZ.toUTC.apply(TZ, t.inputs);
+    if (t.result !== result.toISOString()) {
+        console.log(result);
+        throw new Error(
+            `Invalid TZ to UTC conversion for ${t.desc}:\n` +
+                `\tExpected: ${t.result}\n` +
+                `\tActual: ${result.toISOString()}\n`
+        );
+    }
+}
+
 // At this real UTC time, what does the timezone translate it to?
 [
     //
@@ -21,19 +44,23 @@ var TZ = require("./");
 
     // 12:15am NY -0500 => -0400
     {
+        desc: "UTC to 12:15am NY EST",
         inputs: ["2021-03-14T05:15:59.000Z", "America/New_York"],
         result: "2021-03-14T00:15:59.000-0500",
     },
     {
+        desc: "UTC to 12:15am NY EST (2)",
         inputs: ["2021-03-14T00:15:59.000-0500", "America/New_York"],
         result: "2021-03-14T00:15:59.000-0500",
     },
     // 1:15am NY (non-DST)
     {
+        desc: "UTC to 1:15am NY EST",
         inputs: ["2021-03-14T06:15:59.000Z", "America/New_York"],
         result: "2021-03-14T01:15:59.000-0500",
     },
     {
+        desc: "UTC to 1:15am NY EST (2)",
         inputs: ["2021-03-14T01:15:59.000-0500", "America/New_York"],
         result: "2021-03-14T01:15:59.000-0500",
     },
@@ -42,19 +69,23 @@ var TZ = require("./");
 
     // 3:15am NY (DST)
     {
+        desc: "UTC to 3:15am NY EDT",
         inputs: ["2021-03-14T07:15:59.000Z", "America/New_York"],
         result: "2021-03-14T03:15:59.000-0400",
     },
     {
+        desc: "UTC to 3:15am NY EDT (2)",
         inputs: ["2021-03-14T03:15:59.000-0400", "America/New_York"],
         result: "2021-03-14T03:15:59.000-0400",
     },
     // 4:15am NY
     {
+        desc: "UTC to 4:15am NY EDT",
         inputs: ["2021-03-14T08:15:59.000Z", "America/New_York"],
         result: "2021-03-14T04:15:59.000-0400",
     },
     {
+        desc: "UTC to 4:15am NY EDT (2)",
         inputs: ["2021-03-14T04:15:59.000-0400", "America/New_York"],
         result: "2021-03-14T04:15:59.000-0400",
     },
@@ -75,47 +106,57 @@ var TZ = require("./");
 
     // 12:15am NY -0400 => -0500
     {
+        desc: "UTC to 2021 Nov 7, 12:15am NY EDT",
         inputs: ["2021-11-07T04:15:59.000Z", "America/New_York"],
         result: "2021-11-07T00:15:59.000-0400",
     },
     {
+        desc: "UTC to 2021 Nov 7, 12:15am NY EDT (2)",
         inputs: ["2021-11-07T00:15:59.000-0400", "America/New_York"],
         result: "2021-11-07T00:15:59.000-0400",
     },
     // 1:15am NY (DST) -0400
     // NOTE: 1:15am happens TWICE (with different offsets)
     {
+        desc: "UTC to 2021 Nov 7, 1:15am NY EDT",
         inputs: ["2021-11-07T05:15:59.000Z", "America/New_York"],
         result: "2021-11-07T01:15:59.000-0400",
     },
     {
+        desc: "UTC to 2021 Nov 7, 1:15am NY EDT (2)",
         inputs: ["2021-11-07T01:15:59.000-0400", "America/New_York"],
         result: "2021-11-07T01:15:59.000-0400",
     },
     // 1:15am NY (non-DST) -0500
     {
+        desc: "UTC to 2021 Nov 7, 1:15am NY EST",
         inputs: ["2021-11-07T06:15:59.000Z", "America/New_York"],
         result: "2021-11-07T01:15:59.000-0500",
     },
     {
+        desc: "UTC to 2021 Nov 7, 1:15am NY EST (2)",
         inputs: ["2021-11-07T01:15:59.000-0500", "America/New_York"],
         result: "2021-11-07T01:15:59.000-0500",
     },
     // 2:15am NY -0500
     {
+        desc: "UTC to 2021 Nov 7, 2:15am NY EST",
         inputs: ["2021-11-07T07:15:59.000Z", "America/New_York"],
         result: "2021-11-07T02:15:59.000-0500",
     },
     {
+        desc: "UTC to 2021 Nov 7, 2:15am NY EST (2)",
         inputs: ["2021-11-07T02:15:59.000-0500", "America/New_York"],
         result: "2021-11-07T02:15:59.000-0500",
     },
     // 3:15am NY
     {
+        desc: "UTC to 2021 Nov 7, 3:15am NY EST",
         inputs: ["2021-11-07T08:15:59.000Z", "America/New_York"],
         result: "2021-11-07T03:15:59.000-0500",
     },
     {
+        desc: "UTC to 2021 Nov 7, 3:15am NY EST (2)",
         inputs: ["2021-11-07T03:15:59.000-0500", "America/New_York"],
         result: "2021-11-07T03:15:59.000-0500",
     },
@@ -124,22 +165,152 @@ var TZ = require("./");
     // Positive Offset Test
     //
 
-    // 4:15am Colombo +0530 (not DST)
+    // Colombo +0530 (not DST)
     {
+        desc: "UTC to Asia/Colombo (1)",
         inputs: ["2021-03-14T08:15:59.000Z", "Asia/Colombo"],
         result: "2021-03-14T13:45:59.000+0530",
     },
     {
+        desc: "UTC to Asia/Colombo (2)",
         inputs: ["2021-03-14T13:45:59.000+0530", "Asia/Colombo"],
         result: "2021-03-14T13:45:59.000+0530",
     },
-].forEach(function (t) {
-    var result = TZ.fromUTCToTimeZone.apply(TZ, t.inputs).toISOString();
-    if (t.result !== result) {
-        throw new Error(
-            "Invalid Conversion:\n" +
-                `\tExpected: ${t.result}\n` +
-                `\tActual: ${result}\n`
-        );
-    }
-});
+    {
+        desc: "UTC to Asia/Colombo (3)",
+        inputs: ["2021-11-07T08:15:59.000Z", "Asia/Colombo"],
+        result: "2021-11-07T13:45:59.000+0530",
+    },
+    {
+        desc: "UTC to Asia/Colombo (4)",
+        inputs: ["2021-11-07T13:45:59.000+0530", "Asia/Colombo"],
+        result: "2021-11-07T13:45:59.000+0530",
+    },
+].forEach(testUtcToTz);
+console.info("Pass: UTC to TZ for America/New_York and Asia/Colombo");
+
+[
+    //
+    // Start-of-DST Tests
+    //
+
+    // [Start]
+    // What time is '2021-03-14 01:15:59.000 in New York' in UTC? // 2021-03-14 06:15:59.000
+    //                                                            // 2021-03-14T01:15:59.000-0500
+    // What time is '2021-03-14 02:15:59.000 in New York' in UTC? // 2021-03-14 07:15:59.000
+    //                                                            // 2021-03-14T03:15:59.000-0400
+    // What time is '2021-03-14 03:15:59.000 in New York' in UTC? // 2021-03-14 07:15:59.000
+    //                                                            // 2021-03-14T03:15:59.000-0400
+    // What time is '2021-03-14 04:15:59.000 in New York' in UTC? // 2021-03-14 08:15:59.000
+    //                                                            // 2021-03-14T04:15:59.000-0400
+    // [End]
+
+    // 12:15am NY -0500 => -0400
+    {
+        // 2021-03-14T05:15:59.000Z
+        desc: "2021 Mar 14, 12:15am NY EST to UTC",
+        inputs: ["2021-03-14 00:15:59.000", "America/New_York"],
+        result: "2021-03-14T00:15:59.000-0500",
+    },
+    // 1:15am NY (non-DST)
+    {
+        // 2021-03-14T06:15:59.000Z
+        desc: "2021 Mar 14, 1:15am NY EST to UTC",
+        inputs: ["2021-03-14 01:15:59.000", "America/New_York"],
+        result: "2021-03-14T01:15:59.000-0500",
+    },
+
+    // NOTE: Can't 2:15am NY, because it does not exist (skipped by DST)
+    // This test is here to document the "undefined" behavior
+    {
+        // Both 2021-03-14T06:15:59.000Z and 2021-03-14T07:15:59.000Z
+        // would be reasonable substitutions, I think
+        desc: "2021 Mar 14, 2:15am NY ExT to UTC",
+        inputs: ["2021-03-14 02:15:59.000", "America/New_York"],
+        //result: "2021-03-14T01:15:59.000-0500", // 2021-03-14T06:15:59.000Z
+        result: "2021-03-14T02:15:59.000-0400", // 2021-03-14T06:15:59.000Z
+        //result: "2021-03-14T02:15:59.000-0500", // 2021-03-14T07:15:59.000Z
+    },
+
+    // 3:15am NY (DST)
+    {
+        // 2021-03-14T07:15:59.000Z
+        desc: "2021 Mar 14, 3:15am NY EDT to UTC",
+        inputs: ["2021-03-14 03:15:59.000", "America/New_York"],
+        result: "2021-03-14T03:15:59.000-0400",
+    },
+    // 4:15am NY
+    {
+        // 2021-03-14T08:15:59.000Z
+        desc: "2021 Mar 14, 4:15am NY EDT to UTC",
+        inputs: ["2021-03-14 04:15:59.000", "America/New_York"],
+        result: "2021-03-14T04:15:59.000-0400",
+    },
+
+    //
+    // End-of-DST Tests
+    //
+
+    // [Start]
+    // What time is '2021-11-07 01:15:59.000 in New York' in UTC? // 2021-11-07 05:15:59.000
+    //                                                            // 2021-11-07T01:15:59.000-0400
+    //                                                            // 2021-11-07 06:15:59.000
+    //                                                            // 2021-11-07T01:15:59.000-0500
+    // What time is '2021-11-07 02:15:59.000 in New York' in UTC? // 2021-11-07 07:15:59.000
+    //                                                            // 2021-11-07T02:15:59.000-0500
+    // What time is '2021-11-07 03:15:59.000 in New York' in UTC? // 2021-11-07 08:15:59.000
+    // [End]
+
+    // 12:15am NY -0400 => -0500
+    {
+        // 2021-11-07T04:15:59.000Z
+        desc: "2021 Nov 7, 12:15am NY EDT to UTC",
+        inputs: ["2021-11-07 00:15:59.000", "America/New_York"],
+        result: "2021-11-07T00:15:59.000-0400",
+    },
+
+    // 1:15am NY (DST) -0400
+    // NOTE: 1:15am happens TWICE (with different offsets), so we skip one
+    {
+        // ==> 2021-11-07T05:15:59.000Z
+        // [Skip] 2021-11-07T06:15:59.000Z
+        desc: "2021 Nov 7, 1:15am NY ExT to UTC",
+        inputs: ["2021-11-07 01:15:59.000", "America/New_York"],
+        result: "2021-11-07T01:15:59.000-0400", // 2021-11-07T05:15:59.000Z
+        //result: "2021-11-07T01:15:59.000-0500", // 2021-11-07T06:15:59.000Z
+    },
+
+    // 2:15am NY -0500
+    {
+        // 2021-11-07T07:15:59.000Z
+        desc: "2021 Nov 7, 2:15am NY EST to UTC",
+        inputs: ["2021-11-07 02:15:59.000", "America/New_York"],
+        result: "2021-11-07T02:15:59.000-0500",
+    },
+    // 3:15am NY
+    {
+        // 2021-11-07T08:15:59.000Z
+        desc: "2021 Nov 7, 3:15am NY EST to UTC",
+        inputs: ["2021-11-07T03:15:59.000", "America/New_York"],
+        result: "2021-11-07T03:15:59.000-0500",
+    },
+
+    //
+    // Positive Offset Test
+    //
+
+    // Colombo +0530 (not DST)
+    {
+        // 2021-03-14T08:15:59.000Z
+        desc: "Asia/Colombo to UTC (1)",
+        inputs: ["2021-03-14 13:45:59.000", "Asia/Colombo"],
+        result: "2021-03-14T13:45:59.000+0530",
+    },
+    {
+        // 2021-03-14T08:15:59.000Z
+        desc: "Asia/Colombo to UTC (2)",
+        inputs: ["2021-11-07 13:45:59.000", "Asia/Colombo"],
+        result: "2021-11-07T13:45:59.000+0530",
+    },
+].forEach(testTzToUtc);
+console.info("Pass: TZ to UTC for America/New_York and Asia/Colombo");