diff --git a/.gitignore b/.gitignore
index bc2cd3e..5d4e70a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,3 +6,4 @@ build
 .DS_Store
 .editorconfig
 Makefile
+browser-build/fhirpath.min.js.map
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 439a294..ebc773f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,13 @@
 This log documents significant changes for each release.  This project follows
 [Semantic Versioning](http://semver.org/).
 
+## [0.17.5] - 2019-11-18
+### Fixed
+- Although one could add a Quantity to a date, subtracting the quantity resulted
+  in an error.
+- Fixed functions toDateTime() and toTime(), for the minified versions of the
+  code.
+
 ## [0.17.4] - 2019-10-22
 ### Fixed
 - Fixed the compile API, so that the returned function now takes the "context"
diff --git a/browser-build/webpack.config.js b/browser-build/webpack.config.js
index ce3cfcb..2e1beca 100644
--- a/browser-build/webpack.config.js
+++ b/browser-build/webpack.config.js
@@ -7,7 +7,7 @@ module.exports = {
   entry: './fhirpathRequire.js',
   mode: 'production',
   // mode: 'development',
-  // devtool: 'source-map',
+  devtool: 'source-map',
   output: {
     path: __dirname,
     filename: './fhirpath.min.js',
diff --git a/demo/test/protractor/spec/json.spec.js b/demo/test/protractor/spec/json.spec.js
index 7f60ba8..54514ba 100644
--- a/demo/test/protractor/spec/json.spec.js
+++ b/demo/test/protractor/spec/json.spec.js
@@ -15,4 +15,15 @@ describe('JSON entry mode', function() {
     expect(cmVal).toContain('"resourceType": "Patient"');  // json
     expect(output.getText()).toEqual('- Jim');
   });
+
+});
+
+describe('minified FHIRPath', function() {
+  // This section tests that the minification process has not broken things.
+  it('should have a working toDateTime()', function () {
+    $('#path').clear();
+    $('#path').sendKeys('birthDate.toDateTime()');
+    var EC = protractor.ExpectedConditions;
+    browser.wait(EC.textToBePresentInElement($('#output'), '1974'), 3000);
+  });
 });
diff --git a/demo/webpack.config.js b/demo/webpack.config.js
index 289424e..4106523 100644
--- a/demo/webpack.config.js
+++ b/demo/webpack.config.js
@@ -7,7 +7,7 @@ module.exports = {
   devServer: {
     contentBase: './build'
   },
-  devtool: 'inline-source-map',
+  devtool: 'source-map',
   module: {
     rules: [
       {
diff --git a/package-lock.json b/package-lock.json
index 4a71f2f..5305b3d 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,6 +1,6 @@
 {
   "name": "fhirpath",
-  "version": "0.17.4",
+  "version": "0.17.5",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {
@@ -8335,9 +8335,9 @@
       }
     },
     "handlebars": {
-      "version": "4.4.0",
-      "resolved": "https://registry.npmjs.org/handlebars/-/handlebars-4.4.0.tgz",
-      "integrity": "sha512-xkRtOt3/3DzTKMOt3xahj2M/EqNhY988T+imYSlMgs5fVhLN2fmKVVj0LtEGmb+3UUYV5Qmm1052Mm3dIQxOvw==",
+      "version": "4.5.3",
+      "resolved": "https://registry.npmjs.org/handlebars/-/handlebars-4.5.3.tgz",
+      "integrity": "sha512-3yPecJoJHK/4c6aZhSvxOyG4vJKDshV36VHp0iVCDVh7o9w2vwi3NSnL2MMPj3YdduqaBcu7cGbggJQM0br9xA==",
       "dev": true,
       "requires": {
         "neo-async": "^2.6.0",
@@ -13882,20 +13882,20 @@
       "dev": true
     },
     "uglify-js": {
-      "version": "3.6.0",
-      "resolved": "https://registry.npmjs.org/uglify-js/-/uglify-js-3.6.0.tgz",
-      "integrity": "sha512-W+jrUHJr3DXKhrsS7NUVxn3zqMOFn0hL/Ei6v0anCIMoKC93TjcflTagwIHLW7SfMFfiQuktQyFVCFHGUE0+yg==",
+      "version": "3.6.9",
+      "resolved": "https://registry.npmjs.org/uglify-js/-/uglify-js-3.6.9.tgz",
+      "integrity": "sha512-pcnnhaoG6RtrvHJ1dFncAe8Od6Nuy30oaJ82ts6//sGSXOP5UjBMEthiProjXmMNHOfd93sqlkztifFMcb+4yw==",
       "dev": true,
       "optional": true,
       "requires": {
-        "commander": "~2.20.0",
+        "commander": "~2.20.3",
         "source-map": "~0.6.1"
       },
       "dependencies": {
         "commander": {
-          "version": "2.20.1",
-          "resolved": "https://registry.npmjs.org/commander/-/commander-2.20.1.tgz",
-          "integrity": "sha512-cCuLsMhJeWQ/ZpsFTbE765kvVfoeSddc4nU3up4fV+fDBcfUXnbITJ+JzhkdjzOqhURjZgujxaioam4RM9yGUg==",
+          "version": "2.20.3",
+          "resolved": "https://registry.npmjs.org/commander/-/commander-2.20.3.tgz",
+          "integrity": "sha512-GpVkmM8vF2vQUkj2LvZmD35JxeJOLCwJ9cUkugyk2nuhbv3+mJvpLYYt+0+USMxE+oj+ey/lJEnhZw75x/OMcQ==",
           "dev": true,
           "optional": true
         },
diff --git a/package.json b/package.json
index cd27329..6b4d205 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "fhirpath",
-  "version": "0.17.4",
+  "version": "0.17.5",
   "description": "A FHIRPath engine",
   "main": "src/fhirpath.js",
   "dependencies": {
diff --git a/src/fhirpath.js b/src/fhirpath.js
index 1c57437..3b97699 100644
--- a/src/fhirpath.js
+++ b/src/fhirpath.js
@@ -130,7 +130,7 @@ engine.invocationTable = {
   "inOp":       {fn: collections.in,  arity: {2: ["Any", "Any"]}},
   "&":          {fn: math.amp,     arity:  {2: ["String", "String"]}},
   "+":          {fn: math.plus,    arity:  {2: ["Any", "Any"]}, nullable: true},
-  "-":          {fn: math.minus,   arity:  {2: ["Number", "Number"]}, nullable: true},
+  "-":          {fn: math.minus,   arity:  {2: ["Any", "Any"]}, nullable: true},
   "*":          {fn: math.mul,     arity:  {2: ["Number", "Number"]}, nullable: true},
   "/":          {fn: math.div,     arity:  {2: ["Number", "Number"]}, nullable: true},
   "mod":        {fn: math.mod,     arity:  {2: ["Number", "Number"]}, nullable: true},
diff --git a/src/math.js b/src/math.js
index 2ebfd3c..b202b04 100644
--- a/src/math.js
+++ b/src/math.js
@@ -34,6 +34,7 @@ engine.amp = function(x, y){
 };
 
 //HACK: for only polymorphic function
+//  Actually, "minus" is now also polymorphic
 engine.plus = function(xs, ys){
   if(xs.length == 1 && ys.length == 1) {
     var x = xs[0];
@@ -51,8 +52,16 @@ engine.plus = function(xs, ys){
   throw new Error("Can not " + JSON.stringify(xs) + " + " + JSON.stringify(ys));
 };
 
-engine.minus = function(x, y){
-  return x - y;
+engine.minus = function(xs, ys){
+  if(xs.length == 1 && ys.length == 1) {
+    var x = xs[0];
+    var y = ys[0];
+    if(typeof x == "number" && typeof y == "number")
+      return x - y;
+    if(x instanceof FP_TimeBase && y instanceof FP_Quantity)
+      return x.plus(new FP_Quantity(-y.value, y.unit));
+  }
+  throw new Error("Can not " + JSON.stringify(xs) + " - " + JSON.stringify(ys));
 };
 
 
diff --git a/src/misc.js b/src/misc.js
index 72e87f4..2c4042f 100644
--- a/src/misc.js
+++ b/src/misc.js
@@ -4,8 +4,6 @@
 
 var util = require("./utilities");
 var types = require("./types");
-var FP_DateTime = types.FP_DateTime;
-var FP_Time = types.FP_Time;
 
 var engine = {};
 
@@ -73,23 +71,23 @@ engine.toString = function(coll){
 
 /**
  *  Defines a function on engine called to+timeType (e.g., toDateTime, etc.).
- * @param timeType a class (contsructor) for a time type (e.g. FP_DateTime).
+ * @param timeType The string name of a class for a time type (e.g. "FP_DateTime").
  */
 function defineTimeConverter(timeType) {
-  let timeName = timeType.name.slice(3);
+  let timeName = timeType.slice(3); // Remove 'FP_'
   engine['to'+timeName] = function(coll) {
     var rtn = [];
     if (coll.length > 1)
       throw Error('to '+timeName+' called for a collection of length '+coll.length);
     if (coll.length === 1) {
-      var t = timeType.checkString(coll[0]);
+      var t = types[timeType].checkString(coll[0]);
       if (t)
         rtn[0] = t;
     }
     return rtn;
   };
 }
-defineTimeConverter(FP_DateTime);
-defineTimeConverter(FP_Time);
+defineTimeConverter('FP_DateTime');
+defineTimeConverter('FP_Time');
 
 module.exports = engine;
diff --git a/test/cases/6.6_math.yaml b/test/cases/6.6_math.yaml
index a13148f..cada6d2 100644
--- a/test/cases/6.6_math.yaml
+++ b/test/cases/6.6_math.yaml
@@ -143,6 +143,8 @@ tests:
       result: ['2016-02-29']
     - expression: "@2016-02-28 + 2 days"
       result: ['2016-03-01']
+    - expression: "@2016-02-28 - 2 days"
+      result: ['2016-02-26']
     - expression: "@2016-01-01 + 1 month"
       result: ['2016-02-01']
     - expression: "@T09:45:23 + 2 years"
diff --git a/webpack.config.js b/webpack.config.js
index 6395c49..0673947 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -10,6 +10,7 @@ module.exports = {
     ]
   },
   mode: 'production',
+  devtool: 'source-map',
   output: {
     path: __dirname,
     filename: 'build/fhirpath.js',