The time of our Solaris server now is slowly more 20 seconds.
How can we change it ?

I use "ntpdate" to synchronise the time time an internal source.

This can be done one off, or repeatedly as a service.

one off:

man -k nntp should do you,

The date command allows you to slowly correct the time, manually, without a connection to the net or to a local time server. It's a one time fix.

using nntp is a permanent solution if you have access to a time server.