That's hit the spot!
I can now sync the deadline date with the end date of the task.
I've even made myself a macro to change a date from all those numbers to something a bit more readable!
Heavily inspired (copy-pasted.) from Eric's macro in the thread linked above.
I'm sure the button can work better as it needs two presses! I know, TWO!!!
or can the button be removed entirely?!
\define readable_time()
\whitespace trim
<!-- split time into parts -->
<$vars yyyy={{{ [<time>split[]first[4]join[]] }}}>
<$vars mm={{{ [<time>split[]first[6]last[2]join[]] }}}>
<$vars dd={{{ [<time>split[]first[8]last[2]join[]] }}}>
<$vars hh={{{ [<time>split[]first[10]last[2]join[]] }}}>
<$vars min={{{ [<time>split[]first[12]last[2]join[]] }}}>
<!-- Rearange time -->
<$vars mm={{{ [<mm>pad[2]] }}} dd={{{ [<dd>pad[2]] }}} hh={{{ [<hh>pad[2]] }}} min={{{ [<min>pad[2]] }}}>
{{{ [<dd>addsuffix[-]addsuffix<mm>addsuffix[-]addsuffix<yyyy>addsuffix[ at ]addsuffix<hh>addsuffix[:]addsuffix<min>] }}}
\end
And the button
<$button> Set Deadline
<$vars time={{ ORIGIN}} offset="-07:29">
<$wikify name="newtime" text=<<adjust_time>>>
<$action-setfield timeline.end=<<newtime>> />
<$action-setfield timeline.start=<<newtime>> />
</$wikify>
</$vars>
<$vars time={{!!timeline.end}}>
<$wikify name="readtime" text=<<readable_time>>>
<$action-setfield deadline=<<readtime>> />
</$wikify>
</$vars>
</$button>