locking formula reference

[bob engleman]

formula is

=IF(last!W22>0,SUM(last!X22:AC22),SUM(last!Q22:V22,last!
X22:AC22))

need to change name of 'last' sheet every two weeks
to date reference for files

in new sheet formula changes to date reference
not ok
in old sheet formula changes to date reference
ok
what is my best strategy