burn.jl uses TimeZones.jl but TimeZones.jl may change behaviour

[iamed2]

There may be some differences between the behaviour of the GC in the example as reported in JuliaLang/julia#40644 and as is now due to a couple of the fields becoming isbits. As ZonedDateTime might also change again in the future as well, it might be worth making a synthetic struct for testing in burn.jl instead of using ZonedDateTime.

[oscardssmith]

we could just pin the version, but making the synthetic benchmark is almost as easy.