Merge field code for Date Calculation

You can combine two of Paul's examples from his excellent tutorial to give you the required result. The first part determines the end of the month from the mergefield date and the second adds 45 days to that.

{SET StartDate {QUOTE {SET Subtract 0} {SET yy{MERGEFIELD DATE \@ yyyy}} {SET mm{MERGEFIELD DATE \@ M}} {SET dd{=MAX(1,IF((mm=2),28+(MOD(yy,4)=0)+(MOD(yy,400)=0)-(MOD(yy,100)=0),IF((mm=4)+(mm=6)+(mm=9)+(mm=11)+({ MERGEFIELD DATE \@ d}>30)=1,30,31))-Subtract)}} "{dd}-{mm}-{yy}" \@ "dd/MM/yyyy"} } {QUOTE {SET Delay 45} {SET a{=INT((14-{StartDate \@ M})/12)}} {SET b{={StartDate \@ yyyy}+4800-a}} {SET c{={StartDate \@ M}+12*a-3}} {SET d{StartDate \@ 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"}

I am not sure what you mean in your final post as Delay-d does not appear in the listing?

It's in the last field code I posted:

{QUOTE{SET Delay 45}
 ...
{SET d{=({DATE \@ d}=29)*(m=2)+(({DATE \@ d}=28)*(m=2)*(1-((MOD(y,4)=0)+(MOD(y,400)=0)-(MOD(y,100)=0))))+((m=4)+(m=6)+(m=9)+(m=11))*({DATE \@ d}=30)+({DATE \@ d}=31)}}
 ...
{SET jd{=INT((153*c+2)/5)+365*b+INT(b/4)-INT(b/100)+INT(b/400)-32045+Delay-d}}
{...
"{dd}-{mm}-{yy}" \@ "dddd, d MMMM yyyy"}

Try:

{QUOTE{SET Delay 45}
{SET m{MERGEFIELD BeginDate \@ M}}
{SET y{MERGEFIELD BeginDate \@ yyyy}}
{SET a{=INT((14-m)/12)}}
{SET b{=y+4800-a}}
{SET c{=m+12*a-3}}

{SET d{=(28+((MOD(y,4)=0)+(MOD(y,400)=0)-(MOD(y,100)=0)))*(m=2)+((m=4)+(m=6)+(m=9)+(m=11))*30+((m=1)+(m=3)+(m=5)+(m=7)+(m=8)+(m=10)+(m=12))*31}}

{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}" \@ "d MMMM yyyy"}