@btime just prints the result (just like @time). If you want something back you should use @benchmark:
julia> using BenchmarkTools
julia> b = @benchmark rand(2,2);
julia> b
BenchmarkTools.Trial:
memory estimate: 112 bytes
allocs estimate: 1
--------------
minimum time: 40.862 ns (0.00% GC)
median time: 43.809 ns (0.00% GC)
mean time: 46.494 ns (1.74% GC)
maximum time: 430.152 ns (88.88% GC)
--------------
samples: 10000
evals/sample: 988
julia> min = minimum(b)
BenchmarkTools.TrialEstimate:
time: 40.862 ns
gctime: 0.000 ns (0.00%)
memory: 112 bytes
allocs: 1
julia> min.time
40.86234817813765