On the my staking sheet there can either be a formula in the end time column or the duration column but not both.
On the rush & cash sheet the reason that end time returned an error is because D15:D16 are actual dates whereas D13:D14 are text entries.
Change the formula in the end time column to read:
Formula: copy to clipboard 
=IF([@Duration]="","",SUM(IF(ISNUMBER([@[Start Time]]),[@[Start Time]],SUM(DATEVALUE(LEFT([@[Start Time]],6)),TIMEVALUE(RIGHT([@[Start Time]],5)))),[@Duration]))

Combining different types of entries is going to make the formulas complex and difficult to troubleshoot if the need arises at some point in the future.
If there is not a good reason to combine the sheets then I wouldn't.
At some point in the near future the file may become so large that some of the sheets may need to be converted to their own workbook which may be easier if they are not combined.