I've noticed that some¹ logs on hydra have lines like this one:

  phase `install-locale' succeeded after 0.0 seconds

while usually² it is:

  phase `install-locale' succeeded after 0 seconds

The attached patch handles this milliseconds case.

But I wonder why does it happen?  ‘gnu-build’ procedure from (guix build
gnu-build-system) module uses ‘gettimeofday’ to calculate these seconds.
And according to the manual (info "(guile) Time"): «whether true
microsecond resolution is available depends on the operating system».
But why both "0" and "0.0" can be met on hydra?

[1] http://hydra.gnu.org/build/691944/log/raw
[2] http://hydra.gnu.org/build/623040/log/raw