Some simple techniques for working with dates.

Addition and subtraction on dates are conveniently performed as IDN numbers. The root functions DateToIDN and IDNToDate are fast and handy: 

      -/DateToIDN¨(1954 7 5)(1952 9 15) ⍝ days between my and my sister's births
658

But we'll get tired of seeing them everywhere. So let's have an operator. 

      asIDN←{IDNToDate ⍺ ⍺⍺ DateToIDN ⍵}
      7 -⍨ asIDN 1954 7 5 ⍝ 1 week before my sister's birthday
1954 6 28 0

Scalars are often more convenient than triples. We can use ⊥ and ⊤ to switch representations. 

      100⊥1954 7 5
19540705
      0 100 100⊤19540705
1954 7 5
      (100⊥⍣¯1)19540705
1954 7 5
      dual←{⍺(⍺⍺⍣¯1)⍵⍵ ⍺ ⍺⍺ ⍵}
      0 100 100⊤dual {⎕←⍵} 19540705                           ⍝ display before recoding
1954 7 5
19540705
      0 100 100⊤dual {⎕←⍵} 19540705 19520915                  ⍝ my and my sister's birthdates
1954 1952
   7    9
   5   15
19540705 19520915
      0 100 100⊤dual {⎕←⍵} 20070714 19520915                  ⍝ today and my birthdate
2007 1952
   7    9
  14   15
20070714 19520915
      0 10000⊤dual {⎕←⍵} 20070714 19520915                    ⍝ split ccyy mmdd instead of ccyy mm dd
2007 1952
 714  915
20070714 19520915
      0 10000⊤dual {1 1⍉⎕←⍵} 20070714 19520915                ⍝ my birthday this year
2007 1952
 714  915
20070915
      0 10000⊤dual {1 1⍉⎕←⍵} 20071014 19520915                ⍝ but perhaps already passed
2007 1952
1014  915
20070915
      0 10000⊤dual {(2↑2⊃>/⍵)+1 1⍉⎕←⍵} 20071014 19520915      ⍝ my next birthday
2007 1952
1014  915
20080915
      nextbirthday←{0 10000⊤dual {(2↑2⊃>/⍵)+1 1⍉⍵} ⍺ ⍵}       ⍝ as a dyadic fn

      20070714 nextbirthday¨19520915 19540705                 ⍝ my and my sister's next birthdays
20070915 20080705
      Jo SJT KEI Jesus←0705 0915 1217 1225                    ⍝ birth years optional
      20070714 nextbirthday¨Jo SJT KEI Jesus
20080705 20070915 20071217 20071225

      YMDToIDN←{DateToIDN 0 100 100∘⊤⍵}                       ⍝ convert ccyymmdd to IDN
      asIDN←{⊃⍺⍺/YMDToIDN¨⍺ ⍵}                                ⍝ perform ⍺⍺ on args as IDNs
      20070915 -asIDN 20070714                                ⍝ days to my next birthday
63
      20070714{⍺ -⍨asIDN ⍺ nextbirthday ⍵}1225                ⍝ days to Christmas
164

      nb←{⍺+⍳⍵-⍺+1}                                           ⍝ whole numbers between args
      wkday←{(7|⍵)∊⍳5}                                        ⍝ is arg a weekday?

      20070714{+/wkday ⍺ nb asIDN ⍺ nextbirthday ⍵}Jesus      ⍝ count the weekdays to Christmas
116