Mixing Formulas

Hi

I have created a spreadsheet with formulas in some columns which are locked and have pasted the formula for the length of the spreadsheet.

The formula is reliant on the data in Column C which needs to be inputted by user.

Formula is working fine when there is data inputted, on the lines where input has not yet been entered the formula returns the last value for the rest of the lines.

Is there a way I can say if cell C31 is blank show 0, if C31 is complete perform this formula =SUMIF($C$17:$C18,">="&$C18-365,$D$17:$D100)?

Thank you for your guidance