1) " I have back dated the documents 2 days",
" Documents were back dated a month",
" I am going to back date the documents or I am back dating the documents".
2) I think future date should be used as back date,
" I have put a future date on the letter" ,
" I future dated the letter".
3) " He stayed in room lest he disturbed anyone",
"He came early lest I got annoyed",
"Come early so the mother doesn't get annoyed".