Currently (online version) the timing diagram doesn't allow negative time values.
For example @200 marks time point 200, but @-200 generates a parsing error: "syntax error"
Example code:
@startuml
Title this is my title
caption some caption
robust "R-Deci" as RD
concise "R-Algo-0" as RA0
robust "R-Algo-1" as RA1
@500
RD is 0
RA0 is 0
@-100
RD is 1
RA0 is 3
@300
RD is 0
@enduml
The expected behaviour is: the timeline starts at -100 and extends to +500
Edited to add example code.