Textexpander snippet for date formatting with configurable options. Made to run on both text-expander and as node cli for testing.

te-date.js

// Use tab keys to quickly go to OK button

let dateFormat = '%fillpopup:name=dateFormat:default=YYYY-MM-DD:MMM DD, YYYY:DD/MM/YY:MM/DD/YY:none%';
let timeFormat = '%fillpopup:name=timeFormat:HH MM:HH MM SS:default=HH MM ZONE:HH MM SS ZONE:none%';
let hour24 = '%fillpart:name=24 hour format:default=yes%hour24%fillpartend%';
let includeYear = '%fillpart:name=Include year:default=yes%includeYear%fillpartend%';
// keeping these at the top because TextExpander has to show the
// full function code every time.
// This is very long code. Use tab keys to quickly go to ok button.
// Time options have spaces instead of colons because I couldn't
// find a way to escape the colons for TextExpander.

// main function
function printDate({dateFormat, timeFormat, hour24, includeYear}) {
  includeYear = Boolean(includeYear);
  hour24 = Boolean(hour24);
  const date = getDate();
  let {zone, dateOptions} = parseDateFormat(date, dateFormat, includeYear);
  let timeOptions = parseTimeFormat(date, timeFormat, hour24);
  const options = {...dateOptions, ...timeOptions};
  return new Intl.DateTimeFormat(zone, options).format(date);
}

// TODO support output to more time zones
// TODO support input from configurable timezone


// function to maintain a single readline interface. this interface
// can be used multiple times but should be generated only once.
// readline will not be available in TextExpander hence it can not
// be required and kept outside all functions
var rl;

// parses the TextExpander syntax of fillpopup to get options list
// this function will only be called from nodejs when testing
function ask(fillStr) {
  const {fillType} = fillStr.match(/^%fill(?<fillType>\w+)/).groups;
  switch (fillType) {
    case 'part':
      return askPart(fillStr);
    case 'popup':
      return askPopup(fillStr);
    default:
      throw new Error('Invalid fill type: ' + fillType);
  }
}

function askPopup(fillStr) {
  const options = fillStr
    .slice(1, -1).split(':').slice(1)
    .filter(o => !o.startsWith('name='))
    .map(o => (o.startsWith('default=') ? o.slice(8) : o));
  if (options.length === 0) throw new Error('empty options list');
  // print an index for each option to terminal to make is easier to select
  // printing it as an object instead of string concat gives it color too
  options.forEach((cv, i) => console.log(i, cv));
  return new Promise(r => {
    // reading from stdin will only give string
    rl.question('Choose one of the above: ', i => r(options[parseInt(i)]));
  });
}

function askPart(fillStr) {
  const [, firstPart, content] = fillStr.split('%');
  const nameKey = firstPart.split(':').find(o => o.startsWith('name='));
  const defaultYes = firstPart.split(':').includes('default=yes');
  const name = nameKey.slice(5) || 'Unnamed optional section';
  return new Promise(r => {
    // reading from stdin will only give string
    rl.question(
      name + ' (y/n) ',
      a => a === 'y' || (defaultYes && a !== 'n') ? r(content) : r('')
    );
  });
}

function getDate() {
  // TextExpander will replace this placeholder with clipboard contents
  // but the contents will not be escaped. Backticks are much less likely
  // to happen in clipboard contents compared to single and double quotes.
  // If it contains a backtick, this code will break and output will give
  // a js error. There doesn't seem a way to make it perfect.
  // Also note that this means that code from clipboard can get executed
  // as javascript if it escapes the backtick.
  let dateInput = `%clipboard`;
  if (dateInput.match(/^\d+$/)) dateInput = parseInt(dateInput);
  // unix timestamps could be in seconds or milliseconds. This condition
  // checks if it is too small, it is likely to be in seconds
  // This means it only work from 1971 to 2968. This is a trade-off to
  // support both seconds and milliseconds.
  if (dateInput < new Date('1971').getTime())
    dateInput = dateInput * 1000;

  let date = new Date(dateInput);
  // if clipboard does not make a date, then take current date
  if (isNaN(date.getTime())) date = new Date();
  return date;
}

// Reference list all date time format options: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat/DateTimeFormat

// javascript does not have a placeholder based date/time formatting
// what it has are locale specific formats and here we have a choice
// of some commonly used formats. There could be more here.
// These locales were might not work in old OS with old JS.
function parseDateFormat(date, format, includeYear) {
  let zone = 'en-IN';
  let dateOptions = {};
  switch (format) {
    case 'YYYY-MM-DD':
      // https://stackoverflow.com/a/58633686/1615721
      zone = 'sv-SE';
      dateOptions = {year: 'numeric', month: '2-digit', day: '2-digit', };
      break;
    case 'MMM DD, YYYY':
      zone = 'en-US';
      dateOptions = {year: 'numeric', month: 'short', day: 'numeric', };
      break;
    case 'DD/MM/YY':
      zone = 'en-IN';
      dateOptions = {year: '2-digit', month: '2-digit', day: '2-digit', };
      break;
    case 'MM/DD/YY':
      zone = 'en-US';
      dateOptions = {year: '2-digit', month: '2-digit', day: '2-digit', };
      break;
  }
  if (!includeYear) dateOptions.year = undefined;
  return {zone, dateOptions};
}

function parseTimeFormat(date, format, hour24) {
  let timeOptions = {};
  switch (format) {
    case 'HH MM':
      timeOptions = {hour: '2-digit', minute: '2-digit'};
      break;
    case 'HH MM SS':
      timeOptions = {timeOptions: 'medium'};
      timeOptions = {hour: '2-digit', minute: '2-digit', second: '2-digit'};
      break;
    case 'HH MM ZONE':
      timeOptions = {hour: '2-digit', minute: '2-digit', timeZoneName: 'short'};
      break;
    case 'HH MM SS ZONE':
      timeOptions = {hour: '2-digit', minute: '2-digit', second: '2-digit', timeZoneName: 'short'};
      break;
  }
  timeOptions.hour12 = !hour24;
  return timeOptions;
}

// Neither window, nor global is accessible inside TextExpander
// window.TextExpander or global.TextExpander will not work
if (typeof TextExpander !== 'undefined') {
  printDate({dateFormat, timeFormat, hour24, includeYear});
} else {
  // nodejs
  (async () => {
    const {stdin: input, stdout: output} = process;
    if (!rl) rl = require('readline').createInterface({input, output});
    dateFormat = await ask(dateFormat);
    timeFormat = await ask(timeFormat);
    hour24 = await ask(hour24);
    includeYear = await ask(includeYear);
    console.log(printDate({dateFormat, timeFormat, hour24, includeYear}));
    // not closing the interface will not exit the program
    rl.close();
  })();
}