Return the time of the tif data since the epoch

        The time is stored in the "61238" tag.

def get_time(self, idx=0):
        """Return the time of the tif data since the epoch

        The time is stored in the "61238" tag.
        """
        timestr = SingleTifPhasics._get_meta_data(path=self.path,
                                                  section="acquisition info",
                                                  name="date & heure")
        if timestr is not None:
            timestr = timestr.split(".")
            # '2016-04-29_17h31m35s.00827'
            structtime = time.strptime(timestr[0],
                                       "%Y-%m-%d_%Hh%Mm%Ss")
            fracsec = float(timestr[1]) * 1e-5
            # use calendar, because we need UTC
            thetime = calendar.timegm(structtime) + fracsec
        else:
            thetime = np.nan
        return thetime