a serial value to the date
numerical
This is a true statement, but is not a question. Ask again using words that make it clear what you want to know.
first
It stores a date as a number which is the number of days starting on the 1st of January 1900 to the date being stored.
In Excel every cell contains what is seen (value) and what is not seen (formula) and in most cases these are the same thing. When you enter a date in the cell Excel recognizes this and instantly converts the date into a single number whose value represents the number of days from January 1, 1900. You can format the cell to still appear as a date but the formula contains this number which makes it easy to compare or sort against other cells also containing dates.
The NOW function
The NOW function puts in the current date and time into a cell. You enter it like this: =NOW()
The default format style that Excel assigns to all cells in a workbook is the General format. This format displays numbers as they are entered and adjusts automatically based on the content of the cell.
Use conditional formatting to set the condition when you enter a particular date.
The formula is =NOW(). Ensure you format the cell to display the date format you want.
This question is not clear. You can use dates in a formula, but you can not assign a formula value to a date. If you are trying to use dates in formulas, use them like you would any other number. If A1 contains a date, you can show the date six days earlier by using the formula =A1-6.
The TODAY function can enter today's date in Excel. The DATE function can be used to build a date from the day, month and year elements.