The Solaris date command does not have many of the useful features of its GNU equivalent. A workaround is to use Tcl. This requires tclsh to be installed (which tclsh). You can implement this workaround as a function:

tcldate() {
    d=${1:-now}   # input date string
    f=${2:-%c}    # output date format
    echo "puts [clock format [clock scan {$d}] -format {$f}]" | tclsh
}

#!/bin/bash
tcldate() {
    d=${1:-now}   # input date string
    f=${2:-%c}    # output date format
    echo "puts [clock format [clock scan {$d}] -format {$f}]" | tclsh
}
tcldate "$1" "$2"

# date ; tcldate "`date`+1month+12days+17hours+12minutes+30seconds" "%Y-%m-%d %H:%M:%S"
Tuesday, June 28, 2016 08:52:39 AM EDT
2016-08-10 02:05:09