Month to date aggreate KPI query postgresql - postgresql

Question

I'm trying to find a way to obtain the amount of sales made from month to month but only comparing the aggregate total up to the current month's date.
In other words, I want to compare this month's sales up until now, against other months sales up until that month's same day.
Example using today's date as a reference (2016-06-18):
Total sales on January 2016 (*From 2016-01-01 to 2016-01-31*): 1000
Total sales on January 2016 (*From 2016-01-01 to 2016-01-17*): 650
Total sales on February 2016 (*From 2016-02-01 to 2016-01-17*): 670
Total sales on July 2016 (*From 2016-06-01 to current_date - 1*): 680
The structure of my data is as follows:
date sales
2016-01-01 5
2016-01-02 4
2016-01-03 5
2016-01-04 7
.
.
.
When I run the query I would like to have a monthly comparison of the totals mentioned above, so a result that looks like this:
month sales
2016-01 650
2016-02 670
.
.
2016-06 680
So comparing the month-to-date total of each month.
This is what I have tried so far and it seems to be fine, but somehow I always get a small difference whenever I make tests for limit days like the first of each month or the last.
select
order_month,
sum(sales) as sales
from table
where extract (day from order_date::date)
<=
extract (day from current_date::date)
- case when extract (day from current_date::date)=1 then 0 else 1 end
group by 1;