What best practices can be recommended for handling date calculations and comparisons in PHP scripts, especially when dealing with user activity tracking?

When handling date calculations and comparisons in PHP scripts, especially for user activity tracking, it is important to ensure consistency in time zones and use proper date functions to avoid errors. One best practice is to always store dates and times in a standardized format (such as Unix timestamp or ISO 8601) to make calculations and comparisons easier.

// Example of storing user activity timestamp in Unix timestamp format
$user_activity_timestamp = time();

// Example of comparing user activity timestamp with a specific date
$specific_date = strtotime('2022-01-01');
if ($user_activity_timestamp > $specific_date) {
    echo "User activity occurred after 2022-01-01";
} else {
    echo "User activity occurred before or on 2022-01-01";
}