Using %timefilter%: min in transform

[amosd92-zz]

I'm building a Gantt chart using vega and my X1 and X2 are date fields.
It means that the bars in the chart can go over the timeframe I'm looking at.
For example, looking at the last 15 minutes, the graph expends itself to the start time of the process (can be more than 1h).
My solution would be yo create a field with tranform and calculate the max between two fields, startProcessDate and %timefilter%: min.
Is there a way I can do that?