Fix #44 - more concise date formatting.

README.md

@@ -178,7 +178,7 @@ If *columns* is not specified, the list of column names that forms the header ro
 var string = d3.csvFormat(data, ["year", "make", "model", "length"]);
 ```
 
-All fields on each row object will be coerced to strings. If the field value is null or undefined, the empty string is used. If the field value is a Date, the ISO 8601 date format is used per [*date*.toISOString](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toISOString). For more control over which and how fields are formatted, first map *rows* to an array of array of string, and then use [*dsv*.formatRows](#dsv_formatRows).
+All fields on each row object will be coerced to strings. If the field value is null or undefined, the empty string is used. If the field value is a Date, the [ECMAScript date-time string format](https://www.ecma-international.org/ecma-262/9.0/index.html#sec-date-time-string-format) (a subset of ISO 8601) is used: for examples, dates at UTC midnight are formatted as `YYYY-MM-DD`. For more control over which and how fields are formatted, first map *rows* to an array of array of string, and then use [*dsv*.formatRows](#dsv_formatRows).
 
 <a name="dsv_formatBody" href="#dsv_formatBody">#</a> <i>dsv</i>.<b>formatBody</b>(<i>rows</i>[, <i>columns</i>]) [<>](https://github.com/d3/d3-dsv/blob/master/src/dsv.js "Source")
 

src/dsv.js

@@ -33,6 +33,41 @@ function inferColumns(rows) {
   return columns;
 }
 
+function pad(value, width) {
+  var s = value + "", length = s.length;
+  return length < width ? new Array(width - length + 1).join(0) + s : s;
+}
+
+function formatYear(year) {
+  return year < 0 ? "-" + pad(-year, 6)
+    : year > 9999 ? "+" + pad(year, 6)
+    : pad(year, 4);
+}
+
+function formatDate(date) {
+  var hours = date.getUTCHours(),
+      minutes = date.getUTCMinutes(),
+      seconds = date.getUTCSeconds(),
+      milliseconds = date.getUTCMilliseconds();
+  return isNaN(date) ? "Invalid Date"
+      : formatYear(date.getUTCFullYear(), 4) + "-"
+      + pad(date.getUTCMonth() + 1, 2) + "-"
+      + pad(date.getUTCDate(), 2)
+      + (milliseconds ? "T"
+          + pad(hours, 2) + ":"
+          + pad(minutes, 2) + ":"
+          + pad(seconds, 2) + "."
+          + pad(milliseconds, 3) + "Z"
+      : seconds ? "T"
+          + pad(hours, 2) + ":"
+          + pad(minutes, 2) + ":"
+          + pad(seconds, 2) + "Z"
+      : minutes || hours ? "T"
+          + pad(hours, 2) + ":"
+          + pad(minutes, 2) + "Z"
+      : "");
+}
+
 export default function(delimiter) {
   var reFormat = new RegExp("[\"" + delimiter + "\n\r]"),
       DELIMITER = delimiter.charCodeAt(0);
@@ -123,7 +158,7 @@ export default function(delimiter) {
 
   function formatValue(value) {
     return value == null ? ""
-        : value instanceof Date ? value.toISOString()
+        : value instanceof Date ? formatDate(value)
         : reFormat.test(value += "") ? "\"" + value.replace(/"/g, "\"\"") + "\""
         : value;
   }

test/csv-test.js

@@ -320,7 +320,8 @@ tape("csvFormatRows(array) separates lines using Unix newline", function(test) {
 });
 
 tape("csvFormatRows(array) converts dates to ISO 8601", function(test) {
-  test.deepEqual(dsv.csvFormatRows([[new Date(Date.UTC(2018, 0, 1))]]), "2018-01-01T00:00:00.000Z");
+  test.deepEqual(dsv.csvFormatRows([[new Date(Date.UTC(2018, 0, 1))]]), "2018-01-01");
+  test.deepEqual(dsv.csvFormatRows([[new Date(2018, 0, 1)]]), "2018-01-01T08:00Z");
   test.end();
 });