How can I calculate the date preceding a given date in unix?

Question

I have two variables: X and Y.

The value of X will be a date given in the format mmddyy and I want to calculate the date preceding that date

Answer 1

Since the -d option doesn't exists on MacOS X or FreeBSD, here is the answer for them

Retrieving a date relative to current time

date -v -1d

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For your question in particular, you'll need the -j and -f flags to use a specific date

date -v -1d -j -f %m%d%y 091509 +%Y%m%d