Echo how many calendar days have passed between the REFERENCE_DATE and today.

days-since.sh

#!/usr/bin/env sh
# Echo how many calendar days have passed between the REFERENCE_DATE and today.

readonly REFERENCE_DATE='2025-01-01'
readonly SECONDS_PER_DAY=86400

error_exit() {
  echo "$1" >&2
  exit "${2:-1}"
}

get_epoch() {
  date_str="$1"
  case "$(uname)" in
  Darwin)
    date -jf "%Y-%m-%d" "$date_str" +%s 2>/dev/null
    ;;
  Linux)
    date -d "$date_str" +%s 2>/dev/null
    ;;
  *)
    error_exit "Unsupported OS: $(uname)"
    ;;
  esac
}

today="$(date '+%Y-%m-%d')"
sec_ref=$(get_epoch "$REFERENCE_DATE") || error_exit "Failed to parse reference date"
sec_now=$(get_epoch "$today") || error_exit "Failed to parse current date"

day_diff=$(((sec_now - sec_ref) / SECONDS_PER_DAY))
echo "$day_diff day(s) since $REFERENCE_DATE"