Sample CouchDB map functions for working with dates without a library.

custom.js

function(doc){
  // handle a custom format using methods in the date object
  var format = "{getFullYear}/{getMonth}/{getDate}",
      date = new Date(doc.created_at),
      // use a regular expression to replace parts of our format string
      formatted_date = format.replace(/\{(\w+)\}/g, function(str, func){
        var time = date[func]();
        // if `func` is `getMonth` we must offset by one
        if(func === 'getMonth'){
          time += 1;
        }
        return time;
      });
  emit(formatted_date, doc.created_at);
}

dates.js

function(doc){
  // use built-in methods for ISO formatting
  var date = new Date(doc.created_at);
  emit(date.getTime(), date.toISOString());
}

parse.js

function(doc){
  // given a format string as a regex...
  var format = /^(\d{1,4})\/(\d{1,2})\/(\d{1,2})$/;
  if(doc.key){
    // we check if they match
    var match = doc.key.match(format);
    if(match){
      // and use our knowledge of what each capture field should be
      // to create a date object
      var date = new Date(match[1], match[2], match[3]);
      emit(doc.key, date.getTime());
    }
  }
}