I'm a little confused by your question, but essentially, you serve half time. Your math may not take into account that the conduct credits don't accrue until at least four days have been served. Half time is half time.
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Mr. Wienberg is correct. If serving a 90 sentence the typical sentence is suppose to be served in 45 days. However, in California there's crowded jails in many counties so a sentence of 90 days could be as little as a book and release.
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4019 is very confusing, especially when one considers there are multiple ways to interpret it depending upon what legislative enactment applies (Senate Bill 18, Senate Bill 76, Assembly Bill 109, Assembly Bill 117 and Assembly Bill 17). You certainly can get different answers to the same question if the question is not clear.
As to a ninety days sentence, with no credits and for a crime that took place after October 31, 2011, for someone who is entitled to two days credit for each four days served, the prisoner should be released in sixty days.
You are misinterpreting paragraphs (b) and (c). After October 31, 2011, everyone serving a county jail sentence does half time. Sections (b) and (c) explain how we get there. In the first instance for every 4 day period of confinement a prison gets one day off for work credits. In the second it's another 1 day off for good time credits. So if you have a 4 day sentence, do the work you are assigned in jail and behave yourself you only serve 2 actual days. Sixty days is just incorrect in your example. 45 allows for work and good time credits.
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