What do the search date filters mean?

It seems that they do mean what they say; eg, "a week ago" does mean posts 7 days old (or more?)
But this seems a rather useless filter?
Surely, more useful is "within the last 7 days", "within the last month", etc?
In other words, "not older than 1 week", "not older than 1 month", etc.
As in, searching for stuff where "I know I saw this recently - not more than a week ago".
Can we at least have these added?
And there really needs to be something between "a month" and "a year" - at least "a quarter" (3 months) and "half a year" ?
Or just let us enter a date range
