# zfs hold milestone_hold mypool/projects@my_milestone

The "zfs holds" command will list all current snapshots that are protected
this way (-r for a recursive list):

# zfs holds -r mypool

The TIMESTAMP column in the output of the above command is from when the
hold was created, not the snapshot it holds. The "zfs destroy" command will
echo a "dataset is busy" message on the console when it encounters a hold.
Use "zfs release" to release the hold on the snapshot:

# zfs release milestone_hold mypool/projects@my_milestone

-- Benedict Reuschling <bcr@FreeBSD.org>