from datetime import datetime
TIMESTAMP = "{0:%Y-%m-%dT%H-%M-%S/}".format(datetime.now())
...
train_log_dir = 'logs/train/' + TIMESTAMP
test_log_dir = 'logs/test/'  + TIMESTAMP
megred = tf.summary.merge_all()
with tf.Session() as sess:
  writer_train = tf.summary.FileWriter(train_log_dir,sess.graph)
  writer_test = tf.summary.FileWriter(test_log_dir)  
  ...other code...
  writer_train.add_summary(summary_str_train,step)
  writer_test.add_summary(summary_str_test,step)