how to set the value of my start time?

I'm not quite sure what the second list is for aside from the tryparse, but do you want a whole new list every time you hit that suggested time, or just update the old list?

Anyways, I'd do something like,

float timestart = 0; timestart += Time.DeltaTime;

if timestart > timevarhere && timestart < timevar+1.2 (just to make sure it's not constantly doing it) { stuff here }

or if you want it to be neater int varint = 0;

if timestart > timevarhere && varint = 0 { stuff here var += 1; } Which will ensure it will only do it once.

Perhaps something like this, where you add a startTime variable to offset the elapsed time and fast forward the simulation in Start according to your (hidden to us) logic.

 using System.Collections.Generic;
 using UnityEngine;
 
 public class CSVRead : MonoBehaviour
 {
     // Change this to 100 if you want to start at 100th second.
     public float startTime = 0.0f;
 
     public class CSVData
     {
         public int simsec;
         public int vnum;
         public string vtype;
         public float cfx;
         public float cfy;
         public float cfz;
 
     }
     List<CSVData> csv = new List<CSVData>();
 
     public void Start()
     {
         SetupCSV();
         FastForwardCSV(startTime);
     }
 
     void SetupCSV()
     {
         TextAsset traffic = Resources.Load<TextAsset>("traff");
         string[] data = traffic.text.Split(new char[] { '\n' });
         Debug.Log(data.Length);
         for (int i = 25; i < data.Length - 1; i++)
         {
             string[] row = data[i].Split(new char[] { ',' });
             CSVData c = new CSVData();
             int.TryParse(row[0], out c.simsec);
             int.TryParse(row[1], out c.vnum);
             c.vtype = row[2];
             float.TryParse(row[3], out c.cfx);
             float.TryParse(row[4], out c.cfy);
             float.TryParse(row[5], out c.cfz);
             csv.Add(c);
         }
     }
 
     void FastForwardCSV(float startTime)
     {
         foreach (var c in csv)
         {
             if (c.simsec < startTime)
             {
                 // Add code to move object to wherever it needs to be
             }
         }
     }
 
     void Update()
     {
         float time = Time.time + startTime;
         // Your ususal code, using time
     }
 }