Formats a JavaScript Date into a local timestamp string: YYYY-MM-DDTHH:MM:SS.mmm.

formatdate.js

/**
 * Formats a Date object into a string of the form:
 *   YYYY-MM-DDTHH:MM:SS.mmm
 *
 * All numeric components are zero-padded to ensure fixed width.
 * The result reflects the local time of the supplied Date object
 * and does not include any timezone suffix.
 *
 * @param {Date} dt - The Date object to format.
 * @returns {string} The formatted timestamp string.
 */
function formatDate(dt) {
  const pad = (n) => String(n).padStart(2, '0');
  const ms = String(dt.getMilliseconds()).padStart(3, '0');
  return `${dt.getFullYear()}-${pad(dt.getMonth() + 1)}-${pad(dt.getDate())}` +
    `T${pad(dt.getHours())}:${pad(dt.getMinutes())}:${pad(dt.getSeconds())}.${ms}`;  
}