Add date_utils lib (https://github.com/espruino/BangleApps/pull/1753)

modules/date_utils.js

@@ -0,0 +1,39 @@
+/* Utility functions that use the 'locale' module so can produce text
+in the currently selected language. */
+
+/** Return the day of the week (0=Sunday)
+    short==0/undefined -> "Sunday"
+    short==1           -> "Sun"
+*/
+exports.getDOW = (dow, short) => require("locale").dow({getDay:()=>dow},short);
+
+/** Return the month (1=January)
+    short==0/undefined -> "January"
+    short==1           -> "Jan"
+*/
+exports.getMonth = (month, short) => require("locale").month({getMonth:()=>month-1},short);
+
+/** Return all 7 days of the week as an array ["Sunday","Monday",...].
+    short==0/undefined -> ["Sunday",...
+    short==1           -> ["Sun",...
+    short==2           -> ["S",...
+*/
+exports.getDOWs = (short) => {
+  var locale = require("locale");
+  var days = [];
+  for (var i=0;i<7;i++)
+    days.push(locale.dow({getDay:()=>i},short).slice(0,(short==2)?1:100));
+  return days;
+}
+
+/** Return all 12 months as an array ["January","February",...]
+    short==0/undefined -> ["January",...
+    short==1           -> ["Jan",...
+*/
+exports.getMonths = (short) => {
+  var locale = require("locale");
+  var months = [];
+  for (var i=0;i<12;i++)
+    months.push(locale.month({getMonth:()=>i},short));
+  return months;
+}