$ cat test
20180618:
20180619:
20180620:
20180621:
20180622:
20180623:
20180624:

$ grep 201806{19..21} test
grep: 20180619: No such file or directory
grep: 20180620: No such file or directory
grep: 20180621: No such file or directory

$ grep 201806* test
zsh: no matches found: 201806*