I would think that the question relates to good time/work time credits under 4019. You should contact your lawyer who can tell you, based on the precise nature of your commitment, your custody credits in the county jail.
The short version is that last year, the legislature amended 4019 to allow half time credit for most people in county jail. It was poorly drafted and gave rise to a number of appellate cases which sought to interpret the language of the statute.
Then, this Fall, the governor signed into law another measure which repealed the latter section and made some other provisions for custody credits. There are still unresolved questions about retroactivity. The oversimplification is that the new statute removed half time credits from people serving county sentences and returned them to one third credits. The half time credits are still available for most people sentenced to state prison once they get to the CDCR and it leaves in place restrictions for 15 percent cases.
Without knowing your particulars, there is no way to answer the question in your case. Ask your lawyer for legal advice on this as we cannot give legal advice but can only generally educate and send you back to your lawyer. Good luck!
Under Penal Code section 4019, you're going to do 2/3 of your sentence. However, in San Bernardino County, you may be eligible for a work program, work release or other alternative to jail. It'll depend on the charge, how much time you have to do, etc. You also can't have been excluded from these options as part of your sentence.
If you had an attorney for your case, discuss this with them. If you didn't have an attorney, get to the jail early on the day you're supposed to report and see what your options are.