Mindate or firstdate crossing with a filter

[SouSou_Paris]

Hi 

I am desperate to find a solution despite the various answers found on this forum:

I have a "date" table => time[Date]
I have an "agent" table => Agent[Agent]
I have a "volume" measure => '_01-Agent Work Measures'[Nb Workflowstep]

=> it means that I cross my agents who have handled X volumes (Nb Workflowstep) at a given date.

I would like to know for each agent, the first date on which it has volumes.

I have tested several formulas which all give me the same result (i.e. my oldest date in my dataset).

Tested :

1/ date min = min('Time'[Date])
Works without filter context, my earliest date is 1/01/2020.
Does not work for my Agent XXXXX  for whom the date should be 21/09/2020.

2/ CALCULATE([date min], filter(Agent, Agent[Agent] = "Name of Agent"))
Returns 1/01/2020 => false
Should return the earliest date for this agent.

3/ CALCULATE(FIRSTDATE('Time'[Date]), FILTER('Agent','_01-Agent Work Measures' [Nb Workflowstep]>50))
Returns 1/01/2020 => false
Should return the date from the moment I have a value > 50

4/ CALCULATE(FIRSTDATE('Time'[Date]),ALLSELECTED('Time'[Date]), FILTER(Agent, '_01-Agent Work Measures'[Nb Workflowstep]>50))
Returns 1/01/2020 => false
Should return the date from the moment I have a value > 50

5/ CALCULATE([date min],FILTER('Agent','_01-Agent Work Measures'[Nb Workflowstep]>100))
Returns 1/01/2020 => false
Should return the date from the moment I have a value > 100

Here is a screenshot.

Let me know if you need further details : 

Many thanks in advance

Sou