```ts
export class LruCache<K, V> {
    private capacity: number;
    private cache: Map<K, { value: V; timestamp: number }>;

    constructor(capacity: number) {
        if (capacity <= 0 || !Number.isInteger(capacity)) {
            throw new Error("Capacity must be a positive integer.");
        }
        this.capacity = capacity;
        this.cache = new Map();
    }

    get(key: K): V | undefined {
        const entry = this.cache.get(key);
        if (entry) {
            entry.timestamp = Date.now(); // Update timestamp
            return entry.value;
        }
        return undefined;
    }

    set(key: K, value: V): void {
        if (this.cache.has(key)) {
            this.cache.set(key, { value, timestamp: Date.now() });
        } else if (this.cache.size >= this.capacity) {
            // Evict the least recently used entry
            const oldestKey = Array.from(this.cache.entries())
                .reduce((oldest, [k, v]) => v.timestamp < oldest.timestamp ? k : oldest, '');
            delete this.cache[oldestKey];
        }
        this.cache.set(key, { value, timestamp: Date.now() });
    }

    get size(): number {
        return this.cache.size;
    }
}
```