Need help with formula that adds two numbers to a date

Hii all . I need help in the formula in attached images. I want to put a formula that will calculate and show me two dates within one cell. Say for eg I have to revise a subject after either 7 or 8 days and I want to make a formula that returns a date after adding 7 and 8 and displays these two values within the same cell. I tried the formula as in Image1 attached herewith
Image1.png.

While it works perfectly for cell D3 and D4, the same formula gives a Value! error on applying in other column
Image2.png.

Kindly help me out.
Thanks

The workbook does not correspond to the pictures so we don't know why those two dates are different formats. Are they dates or strings?

=TEXT(E2+7&DAY(E2),"d/m/yy")&" to "&TEXT(E2+8&DAY(E2),"d/m/yy")

the highlight red is create the error and it seem like need not to put in.

it work for D2 is due to their have the time behind (hour and minute), excel will read hour and minute as decimal point,
the &Day(E2) is add in behind the decimal (eg, 1.00 > 1.0010) , hence it not affect much in the formula, the date still within the same date,

But E2 is have not time value behind the date, excel read as not decimal point behind.
the &Day(E2) is add in behind the whole value (eg 1 > 110),
hence the effect is so significant, as it might over the limit of the calender and cause #Value!.

you would more clear when you go to
Formula (Ribbon) > Formula Auditing (section) > Evaluate formula > Evaluate