NO NO NO!!!
The first day of entry is counted as day1 and the day of exit as the last day. Passport will be stamped/digitally recorded on entry to and exit from the Schengen Zone to ensure compliance. The EU provides an online calculator to help in this regard.
In 2021 you can travel to the Schengen Area for a maximum of 90 days in any 180 days rolling period (so there must be an absence of 90 days if you stay the full 90 days). In practice this means that any time you wish to enter the Schengen, you need to count backwards the last 180 days, and see if you have been present in the Schengen for more than 90 days throughout that period.
So @DaveCh in direct answer to your supposition that you can back to back July - Dec and Jan - June to stay for a year is absolute bollocks! You just can’t have you cake and eat it!!
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