To set computer time manually in FreeBSD is basically easy to do, set date as root with two digits every section
Sections are year, month , day and minute.
An another way is to use a script which makes it with graphical dialogs.
If you give illegal numbers, time does not change.
Of course an another way to set time is to use NTP time.
When using NTP time, the offset cannot be too large, because if offset is too large, NTP refuses to accept the time.
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Set time manually script (requires sudo, yad and xterm packages installed and wheel and sudo groups set)