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You can make it shorter:

<input type="datetime-local" id="cal"> 

function toLocaleIsoString(date) { const localDate = new Date(date - date.getTimezoneOffset() * 60000); //offset in milliseconds. Credit https://stackoverflow.com/questions/10830357/javascript-toisostring-ignores-timezone-offset // Optionally remove second/millisecond if needed localDate.setSeconds(null); localDate.setMilliseconds(null); return localDate.toISOString().slice(0, -1); } window.addEventListener("load", () => { document.getElementById("cal").value = toLocaleIsoString(new Date()); }); 

You can make it shorter:

<input type="datetime-local" id="cal"> 

function toLocalISOString(date) { const localDate = new Date(date - date.getTimezoneOffset() * 60000); //offset in milliseconds. Credit https://stackoverflow.com/questions/10830357/javascript-toisostring-ignores-timezone-offset // Optionally remove second/millisecond if needed localDate.setSeconds(null); localDate.setMilliseconds(null); return localDate.toISOString().slice(0, -1); } window.addEventListener("load", () => { document.getElementById("cal").value = toLocalISOString(new Date()); }); 

You can make it shorter:

<input type="datetime-local" id="cal"> 

window.addEventListener('load', () => { var now = new Date(); now.setMinutes(now.getMinutes() - now.getTimezoneOffset());  /* remove second/millisecond if needed - credit ref. https://stackoverflow.com/questions/24468518/html5-input-datetime-local-default-value-of-today-and-current-time#comment112871765_60884408 */   now.setMilliseconds(null) now.setSeconds(null)   document.getElementById('cal').value = now.toISOString().slice(0, -1); }); 

You can make it shorter:

<input type="datetime-local" id="cal"> 

function toLocaleIsoString(date) { const localDate = new Date(date - date.getTimezoneOffset() * 60000); //offset in milliseconds. Credit https://stackoverflow.com/questions/10830357/javascript-toisostring-ignores-timezone-offset   // Optionally remove second/millisecond if needed localDate.setSeconds(null); localDate.setMilliseconds(null);  return localDate.toISOString().slice(0, -1); } window.addEventListener("load", () => { document.getElementById("cal").value = toLocaleIsoString(new Date()); }); 

You can make it shorter:

<input type="datetime-local" id="cal"> 

window.addEventListener('load', () => { const now = new Date(); now.setMinutes(now.getMinutes() - now.getTimezoneOffset()); document.getElementById('cal').value = now.toISOString().slice(0, -1); }); 

You can make it shorter:

<input type="datetime-local" id="cal"> 

window.addEventListener('load', () => { var now = new Date(); now.setMinutes(now.getMinutes() - now.getTimezoneOffset()); /* remove second/millisecond if needed - credit ref. https://stackoverflow.com/questions/24468518/html5-input-datetime-local-default-value-of-today-and-current-time#comment112871765_60884408 */ now.setMilliseconds(null) now.setSeconds(null)  document.getElementById('cal').value = now.toISOString().slice(0, -1); });