library(stringr)
a = "USER 2020-02-09 01:13SOMETHING INTERESTING HERE.USER 2020-02-10 08:30and something else comes here"
str_match_all(a, "[0-9][0-9]:[0-9][0-9](.*?)(?:\\bUSER|$)")[[1]][, 2]
bGlicmFyeShzdHJpbmdyKQoKYSA9ICJVU0VSICAyMDIwLTAyLTA5IDAxOjEzU09NRVRISU5HIElOVEVSRVNUSU5HIEhFUkUuVVNFUiAgMjAyMC0wMi0xMCAwODozMGFuZCBzb21ldGhpbmcgZWxzZSBjb21lcyBoZXJlIgpzdHJfbWF0Y2hfYWxsKGEsICJbMC05XVswLTldOlswLTldWzAtOV0oLio/KSg/OlxcYlVTRVJ8JCkiKVtbMV1dWywgMl0=