When you run db.getReplicationInfo() then the output is not valid. Result is like this:

{
  configuredLogSizeMB: 990,
  logSizeMB: 990,
  usedMB: 980.21,
  timeDiff: NaN,
  timeDiffHours: NaN,
  tFirst: 'Invalid Date',
  tLast: 'Invalid Date',
  now: 'Tue Mar 21 2023 16:03:39 GMT+0100 (Mitteleuropäische Normalzeit)'
}
 

timeDiff, timeDiffHours, tFirst, tLast are invalid values.

As workaround one can call rs.status() and compare optimeDate of members. However db.getReplicationInfo() should work as intended.

Kind Regards
Wernfried