Getting the current date in bash without spawning a sub-process

Question

This question is pure curiosity. It is easy to get a date by running the date command from bash, but it is an external executable and requires spawning a subpro

Answer 1

bash 4.2 introduced a new specifier for printf; this was extended in bash 4.3 to use the current time if no argument is given. %()T expands to the current time, using the format appearing inside the parentheses.

$ printf '%(%Y-%m-%d_%H:%M:%S)T\n'
2016-03-25_12:38:10