test.sh

#!/bin/bash

read -p "input a date (eg: yyyy_mm_dd_am or pm):" date

echo $date;

if [[ ! $date =~ ^[0-9]{4}_[01][0-9]_[0-3][0-9]_[ap]m$ ]]; then
  echo "Invalid date format!"
  exit 1;
fi

path=$date # maybe you want to do someting here to get full path to target

if [ ! -d "$date" ]; then
  echo "Path does not exists!"
  exit 2;
fi

# validation passed