Is the requirement of a year’s employment with current employer for L-1B calculated as 365 days? How exactly is it measured, at what point in the process does a year need to be completed and is 11 months of employment long enough to qualify?
You have to be eligible for a benefit at the time when a petition is filed. If you are outside the US and your employer is filing a petition, then you must have completed the required year on or before the day when the petition is filed. If you are inside the US and changing to L-1 status, then you must have completed the required year within the 3 years before you entered the US.
1 year does mean 365 days (I am not sure how leap years are handled exactly). Weekends and vacation count toward the requirement.
The 1 year of employment must be "continuous", meaning that there was a continuous period of time when you were employed at the entity abroad, and that during that period of time, you spent at least 365 full days outside the US. "Periods spent in the United States in lawful status for a branch of the same employer or a parent, affiliate, or subsidiary thereof and brief trips to the United States for business or pleasure shall not be interruptive of the one year of continuous employment abroad but such periods shall not be counted toward fulfillment of that requirement." See 8 CFR §214.2(l)(1)(ii)(A). I am not sure how unpaid leave is handled.
If you have questions about very specific details of how the law applies to your case then you should address them to your employer's attorney.