We occasionally see systems with incorrect system time. The system time can either be out by a few minutes or sometimes several days or more.

Accurate timekeeping is a fundamental assumption in an IT system and adverse negative effects can happen if:

1. the system clock is wrong or;
2. (even worse) the clock is changed in big jumps backwards or forwards.

Inaccurate timekeeping generally can have the following negative impacts:

1. Applications that rely on system time can become corrupted (eg. the backup scheduler that sets when it’s meant to run from the current time. If the system clock is in the past and then jumps into the future, it will result in a stale backup as the system thinks it’s meant to run in the past and therefore never runs)
2. System updates or software versions may rely on timestamps to see when it last ran or installed updates, these can get out of sequence and have unknown consequences.
3. File access and modification times can become corrupted and have incorrect modification times. This can cascade into other applications that perform actions based on file modification times (eg. backup all files that have changed since the last backup, if those dates are in the future the files will not be backed up).
4. Data  loss or corruption may occur if a process that makes data pruning decisions based on time are affected. eg. Many (probably all) backup software that performs retention is solely based on the system clock time. If the time jumps into the future or the past, then backups could be pruned prematurely. This can have disastrous consequences if a backup chain has dependent files pruned, thus rendering the backup chain corrupted and unrestorable.