display date by monthly, by monthly & quarterly

using excel 2003.

I have one sheet. In those sheet :
column a : nos of visit
column b : start date
column c to n : 1st visit to 12 visit date

Now i required if column a : nos of visit 12 then date display in each month. If nos of visit 6 then date display by alternet month & if nos of visit 4 then date display after three months gap.

Formula required in column D2 to N4.

Attached file for your ref.

D2: =IF(MOD(COLUMN()-COLUMN($C$1),12/$A2)=0,DATE(YEAR($B2),MONTH($B2)+(COLUMN()-COLUMN($C$1)),DAY($B2)),"")

copy across and down. Format cells as date