HomeyScript for simple date calculation within a flow. The date class is using the timezone configured in the Homey settings. The result is provided as card tags which can be used in cards that come after. Make sure to add delays to the cards that use these tags, because HomeyScripts are executed async.

homeyscript_datum.js

/**
 * Get current date and time as card tags
 * Optional date offset calculation using Homey's timezone
 * suffix: [y]ears, [m]onths, [d]ays, [h]ours, m[i]nutes, [s]econds, [m]illi[s]econds
 * 
 * @example
 * // two months ago
 * '-2m'
 * 
 * @example
 * // 123 days ahead
 * '+123d'
 * 
 * @see {@link https://gist.github.com/fvdm/e5897fd4271e03183d30f5f14805fdcd}
 */

args[0] = args[0] || 0;

const days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
const months = ['jan', 'feb', 'mar', 'apr', 'may', 'jun', 'jul', 'aug', 'sep', 'oct', 'nov', 'dec'];
const str = `${args[0]}`.match( /^(?<offset>[+-]?\d+) ?(?<unit>[ymdhis]|ms)?$/ );
const offset = parseInt( str.groups.offset, 10 );
const unit = str.groups.unit || 'd';

const info = await Homey.system.getInfo();
const local = new Date().toLocaleString( 'en', { timeZone: info.timezone } );
const d = new Date( local );

if ( typeof offset === 'number' ) {
  switch( unit ) {
    case 'y': d.setFullYear( d.getFullYear() + offset ); break;
    case 'm': d.setMonth( d.getMonth() + offset ); break;
    case 'd': d.setDate( d.getDate() + offset ); break;
    case 'h': d.setHours( d.getHours() + offset ); break;
    case 'i': d.setMinutes( d.getMinutes() + offset ); break;
    case 's': d.setSeconds( d.getSeconds() + offset ); break;
    case 'ms': d.setMilliseconds( d.getMilliseconds() + offset ); break;
  }
}

const firstSun = new Date( d.getFullYear(), 0, 0 );
const firstMon = new Date( d.getFullYear(), 0, 1 );
const firstThu = new Date( d.getFullYear(), 0, 4 );

const res = {
  year: d.getFullYear(),
  month: d.getMonth() + 1,
  month3: months[d.getMonth()],
  day: d.getDate(),
  weekday: days[d.getDay()],
  weekday2: days[d.getDay()].substring( 0, 2 ),
  weekSun: Math.floor( ( ( ( d - firstSun ) / 86400000 ) + d.getDay() + 1 ) / 7 ),
  weekMon: Math.floor( ( ( ( d - firstMon ) / 86400000 ) + d.getDay() + 1 ) / 7 ),
  weekThu: Math.floor( ( ( ( d - firstThu ) / 86400000 ) + d.getDay() + 1 ) / 7 ),  
  hours: d.getHours(),
  minutes: d.getMinutes(),
  seconds: d.getSeconds(),
  ms: d.getMilliseconds(),
  ts: d.getTime() / 1000,
};

res.month2 = `${res.month}`.padStart( 2, '0' );
res.day2 = `${res.day}`.padStart( 2, '0' );
res.hours2 = `${res.hours}`.padStart( 2, '0' );
res.minutes2 = `${res.minutes}`.padStart( 2, '0' );
res.seconds2 = `${res.seconds}`.padStart( 2, '0' );
res.date = `${res.year}-${res.month2}-${res.day2}`;
res.time = `${res.hours2}:${res.minutes2}`;

// log( res );

let arr = [];
for ( key in res ) {
  arr.push( tag( `datum_${key}`, res[key] ) );
}

await Promise.allSettled( arr );
return true;