WAL records have timestamps with microsecond precision(6), so --recovery-target-time parameter must support them:

rmgr: Transaction len (rec/tot): 66/ 66, tx: 554, lsn: 0/01840C40, prev 0/01840BA0, desc: COMMIT 2019年08月06日 17:21:14.670041 MSK; inval msgs: snapshot 2396

I think we can just use https://github.com/thatguystone/approxidate
Our license is compatible with GPL