If you're using Bash you could also use one of the following commands:
printf '%(%Y%m%d%H%M%S)T' # prints the current time
printf '%(%Y%m%d%H%M%S)T' -1 # same as above
printf '%(%Y%m%d%H%M%S)T' -2 # prints the time the shell was invoked
You can use the Option -v varname to store the result in $varname instead of printing it to stdout:
printf -v varname '%(%Y%m%d%H%M%S)T'
While the date command will always be executed in a subshell (i.e. in a separate process) printf is a builtin command and will therefore be faster.