Is there a function to do this?
If not and I/someone writes it, is there interest to add it to std.datetime?

Duration t = ...;
t.to!string => 5 secs, 889 ms, and 811 μs
t.round.to!string    => 5 secs

t=...;
t.to!string => 889 ms, and 811 μs
t.round.to!string    => 889 secs

Use case: shorter logs.

Similarly, it would be nice to have a module in phobos for representing other units such as bytes, and have a similar rounding scheme as in unix command 'ls -lh' which shows 532K, 12M etc)