A period expression indicates a span of time, or a reporting interval,
or both.  The full syntax is:

@example
[INTERVAL] [BEGIN] [END]
@end example

The optional @var{INTERVAL} part may be any one of:

@example
every day
every week
every monthly
every quarter
every year
every N days     # N is any integer
every N weeks
every N months
every N quarters
every N years
daily
weekly
biweekly
monthly
bimonthly
quarterly
yearly
@end example

After the interval, a begin time, end time, both or neither may be
specified.  As for the begin time, it can be either of:

@example
from <SPEC>
since <SPEC>
@end example

The end time can be either of:

@example
to <SPEC>
until <SPEC>
@end example

Where @var{SPEC} can be any of:

@example
2004
2004/10
2004/10/1
10/1
october
oct
this week  # or day, month, quarter, year
next week
last week
@end example

The beginning and ending can be given at the same time, if it spans a
single period.  In that case, just use @var{SPEC} by itself.  In that
case, the period @samp{oct}, for example, will cover all the days in
october.  The possible forms are:

@example
<SPEC>
in <SPEC>
@end example

Here are a few examples of period expressions:

@example
monthly
monthly in 2004
weekly from oct
weekly from last month
from sep to oct
from 10/1 to 10/5
monthly until 2005
from apr
until nov
last oct
weekly last august
@end example