Which could be used in relative time referencing :
```
@startuml
@30 as :reservation_1_start
@30 as :reservation_duration
@:reservation_1_start+:reservation_duration as reservation1_end
@enduml
```
Also adding the possibility to chain operations in refferring to relative time :
```
@startuml
@30 as :reservation_1_start
@30 as :reservation_duration
@10 as :reservation_extra_time
@:reservation_1_start+:reservation_duration+:reservation_extra_time as reservation1_end_with_extra_time
@enduml
```