#!/usr/bin/env bash

if [[ -z "$1" ]]; then
    cat << EOF
reldate - Format or adjust dates

Usage: reldate <date|adjustment>

Examples:
  reldate "2025-12-12"    # Output: 2025-12-12
  reldate "+1d"           # Output: tomorrow's date
  reldate "-1w"           # Output: date 1 week ago
  reldate "+2m"           # Output: date 2 months from now
  reldate "-1y"           # Output: date 1 year ago

Adjustments:
  d = days, w = weeks, m = months, y = years
  Use + for future, - for past

EOF
    exit 1
fi

if [[ "$1" =~ ^[0-9]{4}-[0-9]{2}-[0-9]{2}$ ]]; then
    date -j -f "%Y-%m-%d" "$1" "+%Y-%m-%d" 2>/dev/null || echo "Invalid date: $1"
else
    date -v "$1" "+%Y-%m-%d"
fi