BarTime could be called anytime by any plugins and holster just detects one of possible reasons of stopping.
A more reliable way is to not rely on BarTime: Hooking
WeaponIdle of
weapon_c4 and checking
m_bStartedArming.
Code:
void CC4::WeaponIdle(void)
{
if (m_bStartedArming)
{
// If you are here it means you stopped to plant.
}