Description:
I set up a spreadsheet in Excel years ago in which I track certain dates, then I add 7 to figure out the date a week later. For most dates, this gives me a date. But for some, I get a ### answer. 

For example, I have 11/2/2016 in cell F164. The formula =F164+7 yields 11/9/2017. Cell F165 contains 11/21/2016; the formual =F165+7 yields ### . Same with 11/04/17; 12/8/2016; 11/20/2015; 10/3/2014. But with 10/30/2014, I get the correct answer (11/6/2014). 

It seems to only occur with two digits months, though not for all dates in those three months. I have tried copying dates - and the results are the same. I.e., copying the 11/21/2016 date into F164 yields ### in the cell with the formula.

This does *not* happen when I open a new spreadsheet and enter the two dates above! So this must be an Excel conversion issue... Also, if I copy the set of cells, the issue goes away. However, copying everything on the sheet reproduces the issue. 

Workaround: Select a bunch of cells; select Format > Clear Direct Formatting. The reformat with dates format. 

Steps to Reproduce:
1. Other than selecting the whole sheet and copy & pasting it, I was unable to reproduce it. 
2.
3.

Actual Results:  
11/2/2016	11/9/2016
11/21/2016	###


Expected Results:
11/2/2016	11/9/2016
11/21/2016	11/28/2016



Reproducible: Couldn't Reproduce


User Profile Reset: No


OpenGL enabled: Yes

Additional Info:
Version: 5.4.2.2
Build ID: 22b09f6418e8c2d508a9eaf86b2399209b0990f4
CPU threads: 4; OS: Mac OS X 10.13.1; UI render: default; 
Locale: en-US (en_US.UTF-8); Calc: group


User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:56.0) Gecko/20100101 Firefox/56.0