dynamic cell address

justa-user 

First an answer to your question.  The formula you require is

= INDIRECT("Sheet1!D"&A1)

I would, however, never consider using such a formula.  I would consign Excel to the trash can if I were forced to use such a notation, made worse by the presence of a volatile function.  I would apply defined names both to the the column range and the index cell that reflects their business significance rather than their location of the sheet.  The formula would be of the form

= INDEX(ColumnD, recordNum)

For example
= INDEX(balance, period)

That said, I hope INDIRECT works for you.