creating a date range trigger

hi i am currently trying to construct a date based maintainance sheet

i will try to explain what im trying to do

in one cell date is inputted
the next cell automatically adds +30 to generate next test date

what i am trying to do is get the sheet to
there are 4 testers so its a 3 month cycle for each ie
tester 1 jan
tester 2 feb
tester 3 march
tester 4 april
tester 1 may
tester 2 june

you get the picture? i hope
so if


cell A1 todays date

a2...................................b2......................................c2

21 jan.............................21 feb................................tester 2


inputed ...................... =A1+30
and date
formated
cell


i am trying to get the formula to tell me after the inputed date has been changed who the next tester is

i have tried using the IF command but it wont let me specify a date range ie 1 jan to 31 jan (or in the specified date codes) date range only one specific date with >= or <= which doesnt quite work properly

i hope this isnt confusing?