just made some additional ground. Time for a nap.

Next step is to dive into the when to reset logic

check_pf_interface_usage.sh

 #!/bin/sh
 
 status=""
+monthnow=$(date +%m)
+daynow=$(date +%d)
+HHMMnow=$(date +%H%M)
+
 
 exitunknown() {
 	echo "UNKNOWN - $status"
@@ -10,16 +14,51 @@ exitunknown() {
 
 monthlogic() {
 
+case "$monthnow" in
+	02)	periodsecs=2419200;;
+	04 | 06 | 09 | 11) periodsecs=2592000;;
+	01 | 03 | 05 | 07 | 08 | 10 | 12) periodsecs=2678400;;
+esac
+
+dayreset=
+}
+
+
+
+
+daylogic() {
+
+# Basic error checking based on length alone
+if [ $(expr "${resetdate}" : '.*') -ne 5 ]; then
+	status="You seem to have an improperly formatted date for reset."
+	exitunknown
+fi
+
+HHMMreset="$(echo "$resetdate" | cut -c2,3,4,5)"
+echo $HHMMreset
+periodsecs=86400
+
 }
+
+
+
+getnextreset() {
+
+
+
+
+}
+
 # GET ARGS
-while getopts r:i:w: o
+while getopts r:i:w:h o
 do	case "$o" in
 	r)	resetdate="$OPTARG";;
 	i)	interface="$OPTARG";;
 	w)	warning="$OPTARG";;
-	[?])	print >&2 " Usage: $0 -r date -i interface  [-w warning threshold]" \
+	h)	print >&2 " Usage: $0 -r date -i interface  [-w warning threshold]" \
+		"\n\t$0 -h (shows this message)" \
 		"\n\n\tdate should be the time you want the counters to reset" \
-		"\n\tit will use the format <m|d>[DD]HHMM" \
+		"\n\tit will use the format <m|d>[dd]HHMM" \
 		"\n\tso if you want to reset at the start of the month use m010000" \
 		"\n\tor if you want to reset daily at 3:00 AM use d0300" \
 		"\n\twarning threshold is optional and measured in bytes."
@@ -31,7 +70,7 @@ shift $OPTIND-1
 period="$(echo "$resetdate" | cut -c1)"
 case "$period" in
 	m | M )	monthlogic;;
-	d | D ) periodsecs=86400;;
+	d | D ) daylogic;;
 	* ) 	status="You didn't specify a valid date"
 		exitunknown;;
 esac
@@ -43,5 +82,4 @@ if [ $rc -ne 0 ]; then
 	exitunknown
 fi
 
-echo $resetdate
 echo $warning