in the first row you want to calculate put the following formula into k; if
you don't have k-z,aa,ab,ac... you can rightclick to the far right of j's
column and click insert

k1 type in running total or what ever lable you want
k2 formula: =sum(e1:e65536)

sample:
a    b    c    d            e         ..........................
k
                       billed amount
running total
                           5
50
                           10
                           15
                           20

this may be too simple/laborsome, but for the monthly total, you can use the
same formula =sum(e1:e?) in columns l-w or in column k in k4,5; k7,8 and so
onwith the question mark a whole number.

sample:
k                        l                            m
n
running total        march                  april                    may
      50            =sum(e1:e3)      =sum(e4:e6)      =sum(e7:e15)

march
=sum(e1:e3)

april
=sum(e4:e6)

may
=sum(e7:e15)

or you can use the data filter on a date column and periodicaly filter for:
"between 1 month year and 30/31 month year"
you will see all the rows for that date range. you can then calculate
between them and manually put that number in a column/row you like

there may also be a if statment you could do as such:

type month in L1 and result go into L2 and copy accordingly for 11 other
months
i don't know if statements but here is the logic i would try:

if L1 is date range then calculate (e1:e65536) where as (a1:a65536) equals
L1