If you take a look at the PI SDK Programming Reference, you will find:
"Impact of Regional Settings
Because the expression passed to this method and eventually to the server for evaluation is a string, the string formatting of numeric values becomes an issue. Evaluation of a PE syntax string on any PI server assumes that English settings are used - decimals are periods. If an application builds an expression string on a machine set to use non-English regional settings, care must be taken to avoid generating strings that violate the server's assumption. "
From my understanding, although this is related to other regional issues, the logic is that same. You should provide the expression string that the PI Data Archive will process. Therefore, the timestamps provided within the expression should be set according to the PI Server timezone. A solution for that is to convert from UTC to the PI Server timezone before executing IPICalculate methods.
Please refer to this thread to find out some ways to make this conversion.
Hope it helps!