modTimestamp.js

/**
 * Returns a unique GMT timestamp at every specified interval.
 * 
 * Examples:
 * <ul>
 *   <!-- Returns a unique timestamp every 10 minutes -->
 *   <li>var t10m = modTimestamp(600);</li>
 *   <!-- Returns a unique timestamp every 30 seconds -->
 *   <li>var t30s = modTimestamp('30');</li>
 * </ul>
 *
 * Example usage: This function could be used to generate a pseudo-TTL in
 * situations where server-side caching is not possible. Timestamp should
 * be appended to each pseudo-cached file request.
 *
 * Examples:
 * <ul>
 *   <li>var url = 'path/to/file/?' + modTimestamp(60);</li>
 *   <li>var url = 'path/to/file/?ts=' + modTimestamp(60);</li>
 * </ul>
 *
 * @function
 * @public
 * @param {Number|String} interval Time in seconds between unique timestamps
 * @throws {TypeError} interval must be a valid number
 * @throws {Error} interval must be at least one second
 * @returns {Number} Unique GMT timestamp
 * @author David Furfero furf@furf.com
 */
var modTimestamp = function(interval) {
  
  if (isNaN(interval)) {
    throw new TypeError('interval must be a valid number');
  }
  
  if (interval < 1) {
    throw new Error('interval must be at least one second');
  }

  var now       = new Date(),
      tzOffset  = now.getTimezoneOffset() * 60 * 1000,
      gmt       = (new Date(now.getTime() + tzOffset)).getTime(),
      mod       = parseInt(interval, 10) * 1000,
      ts        = gmt - gmt % mod;

  return ts;
};


var t10m = modTimestamp(600);   // Returns a unique timestamp every 10m
var t30s = modTimestamp('30');  // Returns a unique timestamp every 30s

console.log(t10m);
console.log(t30s);