You need to do little more than copy the
Calculate a day, date, month and year, using n days delay example from the tutorial. Having pasted the field into your own document, select it and press Shift-F9 to display the field code. Then all you need do is change the 14 in SET Delay 14 in the field code to 21 and press F9 to update the field. Done. It's that simple.
Code:
{QUOTE
{SET Delay 14}
{SET a{=INT((14-{DATE \@ M})/12)}}
{SET b{={DATE \@ yyyy}+4800-a}}
{SET c{={DATE \@ M}+12*a-3}}
{SET d{DATE \@ d}}
{SET jd{=d+INT((153*c+2)/5)+365*b+INT(b/4)-INT(b/100)+INT(b/400)-32045+Delay}}
{SET e{=INT((4*(jd+32044)+3)/146097)}}
{SET f{=jd+32044-INT(146097*e/4)}}
{SET g{=INT((4*f+3)/1461)}}
{SET h{=f-INT(1461*g/4)}}
{SET i{=INT((5*h+2)/153)}}
{SET dd{=h-INT((153*i+2)/5)+1}}
{SET mm{=i+3-12*INT(i/10)}}
{SET yy{=100*e+g-4800+INT(i/10)}}
"{dd}-{mm}-{yy}" \@ "dddd, d MMMM yyyy"}