I’m using @time to time some code, what does it mean when it returns a negative value? What does it mean when it returns nothing? What is the trusted resolution of @time? I just need to get precision on the level of ~1s.
@time does nothing more than get the current time, run your code, get the current time after that and subtracting the former from the latter (plus some GC/compiler statistics). You can see exactly what it does with @macroexpand.
If it prints a negative time, it would mean that either your computer has its clock running backwards, or a datetime correction in the OS was performed while your code ran.
The timer itself guarantees (in current versions) millisecond resolution, as far as Im aware.
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It’s like you have traveled in time 
The simplest expression
julia> @time 0
0.000001 seconds
even returns a positive result in my case. I have tried it for empty string and as well as nothing. In the documentation of @time it is stated that
│ Note
│
│ For more serious benchmarking, consider the @btime macro from the BenchmarkTools.jl package which among other
│ things evaluates the function multiple times in order to reduce noise.
and it is better to use @btime macro for benchmarking issues.
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Ok I found my mistake, and I feel really silly. My code was something like
time = @time blah()
and I assumed the time variable would hold the time evaluated, but it holds the return value of the function blah! Is there a way to get the result of @time into a variable?
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@time should never be able to report a time of nothing. The actual return value of @time is just the value of the expression and has nothing to do with the time it took to execute. That time is printed but never returned.
julia> a = @time 3-4
0.000001 seconds
-1
julia> a
-1
I suspect you’re trying to read the output of @time into a variable. Perhaps you are looking for @elapsed?
julia> b = @elapsed 3-4
5.0e-7 # this is how long the expression took to execute, in seconds
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