As SkipVought points out, Excel tries to be intuitive. It will retain the format of the first cell it "senses" which needs an applied format. To combat this, I generally recommend the keyboard shortcuts.
Ctrl + Shift + ~ = General
Ctrl + Shift + 1 = Number
Ctrl + Shift + 2 = Time
Ctrl + Shift + 3 = Date
Ctrl + Shift + 4 = Currency
Ctrl + Shift + 5 = Percent
Ctrl + Shift + 6 = Exponent
So I would just hit Ctrl + Shift + ~ or 1 to return to General/Number format.
HTH
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Regards,
Zack Barresse