These docs are for an old release. Info on upgrading to v6
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now

Explicitly sets the “today” date of the calendar. This is the day that is normally highlighted in yellow.

Date, function

Normally, the local browser’s current date is used.

You can override the current date by providing a parsable Date, such as an ISO8601 date string like "2013-12-01T00:00:00".

You can also provide a function that returns a Date.

Overriding the current date is particularly useful when your calendar is using a custom timeZone parameter. The current year/month/date in the custom timezone might be different than the local computer’s current date.

You’ll most likely want to calculate this with a server-side script and inject it into the settings of the calendar. Here is an example in PHP:

<?php
  $datetime = new DateTime('now', 'America/Chicago');
  $datetime_string = $datetime->format('c');
?>

var calendar = new Calendar(calendarEl, {
  now: <?php echo json_encode($datetime_string) ?>
});