Add subheader to a list but only once

I need to add a subheader to a list. Currently every entry with a date that is today gets a subheader but I only want to set it once. The problem seems to be that I can't access the updated value of todaySubheader inside the <span v-if="todaySubheader.

HTML

<template v-for="(meetup, index) in filteredItems">
            <v-subheader
              v-if="checkIsToday(meetup.date)"
              inset
            >
            <span v-if="todaySubheader === false">TODAY</span>
            </v-subheader>

SCRIPT

data () {
    return {
      todaySubheader: false,
[...]
checkIsToday (val) {
  if (val && this.todaySubheader === false) {
    this.todaySubheader = true
  }
  return isToday(new Date(val))
},

How to access the changed value of this.todaySubheader in v-if="todaySubheader?

from Add subheader to a list but only once