Use of named ranges based on cell content

[Intruder]

I have defined a few date named ranges (sub_1, sub_2,...) to be used in a formula to recalculate dates.
How can I use a particular cell value, e.g. value of cell A1=1,in the formula based and construct the formula with correct sub?
Formula, that does not do the job, would be something like '="sub_"&A1 +5' meaning the date calculated should be the date of sub_1 + 5 days
thanx