Converting date string to date object in BSD MacOSX - bash

Question

In mac osx bsd, date -vmon will convert the current date to Monday's date and date +%w displays day of week.
Say, I have input date 9/14/16 in the format mm/dd/yy. How can I convert this string pattern to a date object so that I could perform date functions like -vmon and +%w on it.
I tried the following: startDate=$(date -jf "%m/%d/%y" "9/14/16") and then tried to perform $startDate +"%w" but it doesn't work. I doubt that the startDate is not date and actually String.
How can I convert the string to date so that I perform date manipulation on it?
Edit: The requirement for doing this is: Say an input date is given. Then corresponding to that date, I want to get the beginning and ending working dates of the week i.e Monday's date and Friday's date. Then I want to get next week's Monday's date and Friday's date. How can I do this?

Answer 1

You need to use date twice, once to convert from the string input to an 'epoch time' (the number of seconds since The Epoch — aka 1970-01-01 00:00:00 +00:00), and once to convert an epoch time to an output format of your choosing:
rdate="9/14/16"
epoch_time=$(date -j -f '%m/%d/%y' "$rdate" +'%s')
date -j -vmon -r "$epoch_time"
date -j -vfri -r "$epoch_time" +'%w'
That gave me the output:
Mon Sep 12 10:14:44 PDT 2016
5
Note that Friday is day 5 of the week, so the output is correct. The use of %w when you specify -vdow seems a little moot; you know that the output will be 5. But you can use any other format characters that you need too.
Note that the time portion is defaulted to the current time. You could add 00:00:00 to the converted date and %H:%M:%S to the conversion format (-f argument) to work with midnight, etc.
You can combine it into one command line:
$ date -j -vmon -r $(date -j -f '%m/%d/%y %H:%M:%S' "$rdate 00:00:00" +'%s') +'%Y-%m-%d %H:%M:%S %w %s'
2016-09-12 00:00:00 1 1473663600
$
The last example on the man page for date on Mac OS X mentions the %s format specifier, which is the key to getting this to work. Well, that and using date twice.
Note that the man page gives synopses:
date [-ju] [-r seconds] [-v [+|-]val[ymwdHMS]] ... [+output_fmt]
date [-jnu] [[[mm]dd]HH]MM[[cc]yy][.ss]
date [-jnu] -f input_fmt new_date [+output_fmt]
date [-d dst] [-t minutes_west]
The 'usage' message is less helpful:
usage: date [-jnu] [-d dst] [-r seconds] [-t west] [-v[+|-]val[ymwdHMS]] ...
[-f fmt date | [[[mm]dd]HH]MM[[cc]yy][.ss]] [+format]
The usage message doesn't show that you can't use -f with -r or -v, whereas the manual is clear that you cannot.
Tested: Mac OS X 10.11.6 El Capitan