How to display / show negative time properly in Excel?
Some of us may suffer this problem, when you subtract a later time 12:20 from an earlier time 10:15, you will get the result as ###### error as following screenshots shown. In this case, how could you show negative time properly and normally in Excel?
Display negative time properly with changing Excel's Default Date System
Display negative time properly with Formulas
Convert Time 

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Display negative time properly with changing Excel's Default Date System
Here is an easy and quick way for you to display the negative time normally in Excel by changing the Excel's Default Date System to 1904 date system. Please do as this:
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1. To open the Excel Options dialog box by clicking File > Options in Excel 2010/2013, and clicking Office Button > Excel Options in Excel 2007.
2. Then in the Excel Options dialog box, click Advanced from the left pane, and in the right section, check Use 1904 date system under When calculating this workbook section. See screenshot:
3. After finishing the settings, click OK. And the negative time will be displayed correctly at once, see screenshots:
Display negative time properly with Formulas
If you don’t want to change the date system, you also can use the following formula to solve this task.
Kutools for Excel, with more than 120 handy Excel functions, enhance your working efficiency and save your working time.  
1. Input your dates that you want to calculate them, and enter this formula =TEXT(MAX($A$1:$A$2)MIN($A$1:$A$2),"H::MM") (A1 and A2 indicate the two time cells separately) into a blank cell. See screenshot:
2. Then press Enter key, and you will get the correct result as following shown:
Tip:
Here is another formula also can help you: =IF(A2A1<0, "" & TEXT(ABS(A2A1),"hh:mm"), A2A1)
In this formula, A2 indicates the smaller time, and A1 stands for the larger time. You can change them as you need.
Change Sige of Vlaues (quickly change negative to positive values, or vice versa...)

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