What the difference between the Today and Now filter?

When filtering on date fields, one can choose between a Relative date or a Calendar date.

In case of a Calendar date, the user picks a date using a date picker.

In case of a Relative date, the exact date will be calculated the moment the query executes.
For Relative date, the user can express the date by using either Before/After Now, or Before/After Today.

The Before Now / After Now filter takes the time of query execution and the time component of the date field in the data extension into consideration. For example, if the query executes the 20th of February 2020 at 2pm, and the filter is: Order Date Equals 20 Hours Before Now, all records with Order Date 19th of February 2020 at 6pm will match.

The Before Today / Today / After Today filters look only at the date component of the date field in the data extension, and the day the query executes.
For example, if the query executes at any time the 20th of February 2020, and the filter is: Order Date Equals 1 Day Before Today, all records with Order Date 19th of February 2020 will match, no matter the time of the Order Date.
Note: Time filters Minutes / Hours are not available when using the Today filter.

Note: dates in Salesforce Marketing Cloud are always stored with a time component. If you didn't define a time component when uploading data into a date field, the time is 00:00 (midnight).