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

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

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.