How do I convert a item with local time to UTC?

[rossko57]

@kl8ter

[%1$tH:%1$tM h]

That’s a date time format, it’s not going to work with a Number. It’s just a simple Quantity, like a Number:Temperature, a numeric with units.
[%.1f h]

There’s an idea for fancier formatting here

Number:Time formatting for display

A Number:Time type Item shares the characteristic of other Quantity Types in that it’s state is represented by a single number + unit, as a pair. openHAB can freely work with a variety of units, and convert values like say “90 s” to “1.5 min”, but neither of these are particularly human-friendly - we tend to think in clock-like terms, more like “1 min 30 secs”. There is no standard feature to perform this kind of formatting. However, transformations are very versatile and meant for tasks like…

@kl8ter

Item 'Gmc_brenndauer' received command 1970-01-01T00:46:24.912+0000

It’s no good commanding a Number with a datetime.
You need to send it a Quantity, a single numeric with a unit.
If you want to just update the Item, update it, don’t send a command.
You’re working with milliseconds already, so something like -
Gmc_brenndauer.postUpdate(myResultInMilliseconds.toString + " mS")