Shell Programming and Scripting

Assuming a linux system, bash or ksh, and that that t_date already contains the last day of the month:

Code:

$ echo $t_date
20150531
$ date +%Y%m%d -d @$(expr 86400 + $(date +%s -d $t_date))
20150601

This works for leap years:

Code:

$ t_date=20160228
$ date +%Y%m%d -d @$(expr 86400 + $(date +%s -d $t_date))
20160229
$ t_date=20160229
$ date +%Y%m%d -d @$(expr 86400 + $(date +%s -d $t_date))
20160301

And, if the OP doesn't have a Linux system, but does have a 1993 or later ksh, the following works correctly when $t_date expands to a YYYYMMDD representation of the last day of any month:

Code:

printf '%(%Y%m%d)T\n' "$t_date + 1 day"

And, we could all write code that will work in any POSIX conforming shell on any system if the submitter would only tell us what OS an shell is being used. But, I'm not going to waste my time doing that again until the submitter answers the simple questions I asked in post #2 in this thread:
so we can provide the help the submitter needs with a solution that will work in the submitter's environment.