test => empty substr
test => once
test => not found
test => multi times
test => overlap
test => deny overlap
test => fix bug: deny overlap
refactoring => rename variables
Merge branch 'return_substring_instance_count2' into master
add test into all-tests.html
test => same century
test => different centuries
test => born date less than now date
test => today for default
test => diff date
refactoring => extract diff() simplify code
Merge branch 'calculate_age_in_years' into master
add test into all-tests.html