In essence, this is what you've got:
B5: =TODAY()-31
B6: =TODAY()-95
B7: =TODAY()-6
C5: =TODAY()-B5 (copied down the column)
It's no surprise that column C simply gives you back the numbers you have hard-coded into the column B formula. Where do these numbers 31,95,6 etc come from?
Surely the days outstanding in column C should be a formula based on the invoice due date? At the moment, these are all shown as falling in 2012?
<tbody>
[TD="align: center"]1[/TD]
[TD="align: right"][/TD]
[TD="align: right"][/TD]
[TD="align: right"][/TD]
[TD="align: center"]2[/TD]
[TD="align: right"]14 Apr 2018[/TD]
[TD="align: right"][/TD]
[TD="align: center"]3[/TD]
[TD="align: right"][/TD]
[TD="align: right"][/TD]
[TD="align: right"][/TD]
[TD="align: center"]4[/TD]
[TD="align: right"]Invoice[/TD]
[TD="align: right"]Date[/TD]
[TD="align: right"]Days O/S[/TD]
[TD="align: center"]5[/TD]
[TD="align: right"]1001[/TD]
[TD="align: right"]14 Mar 2018[/TD]
[TD="align: right"]31[/TD]
[TD="align: center"]6[/TD]
[TD="align: right"]1002[/TD]
[TD="align: right"]9 Jan 2018[/TD]
[TD="align: right"]95[/TD]
[TD="align: center"]7[/TD]
[TD="align: right"]1003[/TD]
[TD="align: right"]8 Apr 2018[/TD]
[TD="align: right"]6[/TD]
</tbody>