The following relative date variables can be included as Default Values for Date field type Salesforce Elements in Forms pages:
You may also use the following for Date/Time field types:
These should be used in fields set to be hidden since the value is displayed as the literal text (i.e. "NOW") and only converted to the date format when saved to Salesforce.
NOTE: These options are taken from a longer list of relative formats in PHP. While not officially supported by Soapbox, you may wish to review this list to see if any options may prove useful.