vhdl-modify-date-prefix-string

vhdl-modify-date-prefix-string is a variable defined in `vhdl-mode.el'.
Its value is

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Documentation:
Prefix string of modification date in VHDL file header.
If actualization of the modification date is called (menu,
`M-x vhdl-template-modify'), this string is searched and the rest
of the line replaced by the current date.

You can customize this variable.