SUMIFS Function To Reset SUM if "x" is placed to "0" ??

Hi,

I am trying to figure out how to do a sumif function on column a that has the dates, column b that contains numbers to be added:


A B


January-12-13
January-13-13 500
January-14-13 x
January-15-13 400
January-16-13 600


I would like to have column B added up only from the current date and before the current date up to the last "x".

If today was the 16th of January, I would like to have 600 added with 400 only, because there is an "x" the day before and it will need to stop at the most recent "x".
And: If today was the 16th of January, and column B for today's date contains an x, not a number, I would like that the formula to reset it instead to "0".

A B C
January-12-13 400 400
January-13-13 500 900
January-14-13 x 0
January-15-13 400 400
January-16-13 600 1000

Hi, Thank you for your reply. I have tried the formula and it works. I was wondering if instead a bunch of text is typed in column b, can the number be reset to 0? If so, x would need to be replaced. I would like the option to be able to put in text (no specific text) which resets to zero or also a number which sums up b2 with c1.

Thank you.

Perfect, thank you so much.