datetime conversion of posix and datenum array timezone difference

Hi,

I am converting into datetime arrays some arrays in posix time format (second since 1/1/1970 00:00:00) and in datenum format (days since the year 0/0/0000 00:00:00) using these lines

Given date1 a datenum array and date2 a posix array

date1_conv=datetime(data1, 'ConvertForm', 'datenum', 'TimeZone', 'America/Barbados')
date2_conv= datetime(data2, 'ConvertForm', 'posixtime', 'TimeZone', 'America/Barbados')

I have notice that if I change the TimeZone in the case of posixtime the output change. For example:

date2_conv= datetime(data2, 'ConvertForm', 'posixtime', 'TimeZone', 'UTC')

While the output does not change if I change the timezone in the case of datenum input. For example:

date1_conv= datetime(data1, 'ConvertForm', 'posixtime', 'TimeZone', 'UTC')

My question is why does this happen?

Thanks in advance,

Giacomo

Answer 1

[Walter Roberson]

Posix Time is defined relative to a particular timezone (in the POSIX standard), whereas datenum is not defined relative to any particular timezone.

That said... since POSIX Time is defined relative to UTC, one would expect that datetime would automatically tag on a timezone identifier when you converted from posixtime, but it does not do so in the datetime object.