Design a key-value store that supports storing values with timestamps. You should be able to retrieve the latest value for a key that's less than or equal to a given timestamp.

You're given a string and a list of keywords. Your task is to add bold tags around all occurrences of those keywords in the string.

Convert a non-negative integer into its English word representation. For example, 12345 should be converted to 'Twelve Thousand Three Hundred Forty Five'.