Source time is fixed on UTC. Note that delta T is not utilized in this clock.

Features

Live, real-time Julian Date updated in the browser via JavaScript (PHP supplies the configuration).
Up to 10-digit decimal precision. Choose how many decimal places to display. Computations use 64-bit values, rounded to 10 places.
UTC-based. The Julian Date is calculated from Coordinated Universal Time, not local time.
Configurable label text. Add your own prefix or suffix text (for example, "Julian Date"). Spaces and placement are up to you.
Text before or after. Show the label either before or after the time.
Adjustable update interval. Control how often the clock refreshes, in milliseconds (default 1000 ms).
Optional icon. Show a Font Awesome, IcoMoon, or Glyphicon by entering its CSS class. The icon always appears on the far left.
Pause and resume on hover. The clock pauses while hovered on desktop or touched on mobile, and resumes automatically.
Module class suffix. Style each instance independently with a standard Joomla suffix.
Multiple instances. Place the module in any position as many times as you need. Each instance is self-contained.

How it works
The module renders a small inline JavaScript clock. It computes the Julian Day Number (JDN) from UTC date and time components using the standard Julian-date algorithm, then converts it to the current Julian Date with sub-second precision. PHP is used only to pass your chosen parameters (label, precision, interval, icon, and so on) into the page.

Time basis: UTC (Coordinated Universal Time)
delta T: not applied
Formula: derived from the standard Julian day algorithm

Settings

Clock text - Text shown alongside the date. Include spaces as desired.
Show text before date? - Place the label before or after the time.
Update interval - Refresh frequency in milliseconds (1000 ms = 1 second).
Decimal display - Number of decimal places, 0 to 10 (default 5).
Icon class - Optional CSS class for Font Awesome, IcoMoon, or Glyphicon.
Module class suffix - Extra CSS class to style the module instance.

Compatibility

Joomla 4, 5 & 6
PHP 8.1 or later

Ideal for
Astronomers, observatories, science and education sites, Ham radio and time-keeping displays, or anyone who needs the current Julian Date at a glance.

Notes

Source time is fixed on UTC; local-time conversion is not performed.
delta T is not accounted for in the displayed value.
Display precision is limited to a maximum of 10 decimal places.