To format dates, use syntax similar to the following: {Task.RespondBy(<format>)} where <format> may contain the following

d                   Day of the month (1 - 31)
                     A single-digit day is formatted without a leading zero.
dd                 Day of the month (1 - 31)
                     A single-digit day is formatted with a leading zero.
ddd               Abbreviated name of the weekday (Mon, Tues, Wed, etc.)
dddd             Full name of the weekday (Monday, Tuesday etc.)

M                   Month number (1 – 12)
MM                Month number with leading zero (01 - 12)
MMM              Abbreviated Month Name (e.g., Dec)
MMMM            Full month name (e.g., December)

yy                  Year, formatted with a leading zero (e.g., 2001 is 01)
yyyy               Year, formatted with the full number (e.g., 2001 is 2001)
 
Examples:
{Task.RespondBy}                                                                2018-02-14
{Task.RespondBy(M/d/yy)}                                                   2/14/18
{Matter.IssueDate(M/d/yyyy)}                                              2/14/2018
{Task.FinalDueDate(MM/dd/yyyy)}                                       02/14/2018
{Matter.FilingDate(MMM d, yyyy)}                                         Feb 14, 2018
{Matter.FilingDate(ddd, MMM d, yyyy)}                                 Sun, Feb 14, 2018
{Task.Modified(dddd, MMMM d, yyyy)}                                  Sunday, February 14, 2018