Dates & Times

In VBA dates are represented as sequential whole numbers.
The Date data type supports dates from 1 January 1900 to 31 December 9999.
A whole number represents the number of days since 1 January 1900.
A negative whole number represents dates before 30 December 1899.

In VBA times are represented as decimal fractions.
The Date data type supports times from 0:00:00 to 23:59:59 are supported.
This decimal represents the amount of time that has passed since midnight.

Date - Data Type

If you are working with BOTH date AND time components declare these variables with the Date data type.
The Date data type uses decimal numbers to represent the date AND time.
The Date data type uses 8 bytes.

To access the actual number, convert your Date into a Double using the CDbl conversion function.

Long - Data Type

If you are ONLY WORKING with dates (no time component) you can declare these variables with the Long data type.
Because dates are represented as whole numbers you could use the Long data type as an alternative to using the Date data type.
The reason some people might do this is because the Long data type is only 4 bytes (compared to the Date data type which is 8 bytes).
You cannot use an Integer data type because the whole numbers can be greater than 32,767.
The Long data type uses 4 bytes.

Single - Data Type

If you are ONLY WORKING with times (no date component) you can declare these variables with the Single data type.
Because times are represented as decimal numbers you could use the Single data type as an alternative to using the Date data type.
The reason some people might do this is because the Single data type is only 4 bytes (compared to the Date data type which is 8 bytes).
The Single data type uses 4 bytes.