This depends on how you do your retrieve or count. For example in xpath you can use differen function for local and UTC time. i.e: '[%BeginOfCurrentDayUTC%]' and '[%BeginOfCurrentDay%]'
See also this FAQ from Mendix on DateTime handling: https://docs.mendix.com/refguide/datetime-handling-faq
When, how/by who is this microflow invoked?
When a microflow is triggered as AfterStartup microflow, this microflow is only executed after (re)starting the application. And thus will only run once. Not when a user logs in. Also, the AfterStartUp microflow runs in the context of the app/server. Not in the context of a specific user. This is the reason why you have an offset in DateTime values triggering it manually and as AfterStartUp.
If you want that the tasks are updated each day, then are 2 options (among many others).
In both cases, Hold the DateTime handling in to account, as Maarten mentioned!
When using a scheduled event (which I think you are or should be using) the timezone config can be set through Project Setting > Runtime > Scheduled event time zone.