First day of the month

I have a process that uses the current day and previous day to find files.

currentday=$(date -d "$D" '+%d')
prevday=$(date -d "1 day ago" '+%d')

when this runs on the first of the month it does not return 01 for the current day.  It gave an error 'invalid date'.  When it ran on the second it does show 01 for the previous day.

how do I determine the first day of the month?
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What was the value of $D, and how did it get set?
bjeAuthor Commented:
Did not set this value.
Try setting it to "now" or "today"

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Gerwin Jansen, EE MVETopic Advisor Commented:
To display today's date, this will do, it will display 10 today on 11/10/2015
date '+%d'

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And previous date is displayed like this, as your example is showing already, it will show 09:
date -d "1 day ago" '+%d'

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Your $D variable suggests that you are getting a date from somewhere, are you?

Simulating will show the last date of previous month:
root@hst ~ # date -d "9 days ago" '+%d'
root@hst ~ # date -d "10 days ago" '+%d'

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