How many days left in the current month

try:

in day's left:

=SUMPRODUCT(--(WEEKDAY(ROW(INDIRECT(TODAY()&":"&EOMONTH(TODAY(),0))))={2,3,4,5,6,7}))

in day's gone:

=SUMPRODUCT(--(WEEKDAY(ROW(INDIRECT(EOMONTH(TODAY(),-1)+1&":"&TODAY()-1)))={2,3,4,5,6,7}))

"EOMONTH(TODAY(),-1)+1" translates to "last day of last month + 1" and gives you the first day of the current month.

above formula is @jasonb's; i have just modified it to fit OP's needs.