без году неделя

Etymology
lit. '(for) a week minus/without a year, i.e. an impossibly short or negative duration of time'.

Adverb

 * 1)  for a very short time, less than (normally) required or expected

Usage notes

 * Stressed prepositions are more common in clitics and set expressions.
 * The case of here is partitive. A standard, common and modern genitive case of  is.