I am trying to put together a formula to determine the total values of overdue invoices by the length of their overdue (60+, 90+ and 120+), however I cannot seem to get it quite correct.

I have multiple sheets that this works from and the data n on individual accounts (by monthly invoice) and then summarised on an additional sheet.

The formula that I am using is: =SUMIF('120518667!$C$2:$C$150,"=>"&$E$1-60,'120518667'!$H$2:$H$150) for 60+ days overdue, and with variants of "=>"&$E$1-90, for 90+ days and "=>"&$E$1-120 for 120+ days.
'120518667!$C$2:$C$150, refers to the date; and
'120518667'!$H$2:$H$150 refers to the invoice amount.

However my problem is that my retrieved SUM totals are getting higher rather than lower (i.e the value returned for invoices over 120+ days old is higher than that for invoices over 60+ days).

Where have I gone wrong in my formula?