Get_last1M_of_day in bad data for backtesting

I have to handle with old data which is not 100% clean (german DAX index). Usually trading time is from 8am to 10pm, but in the old data sometimes the trading ends at 5:35 pm, sometimes at 20 pm...

I need a method inside EA in order to know, which Minute of the day is the last one of the day. For this purpose i don't know any function in MQ4. Perhaps you know one. However there is also a possibility to look simply in the csv file (i have exported all data into a 1 Minute ohlc-csv-file).

The function should be called max once per day in order to reduce time for backtesting. For real trading i don't need this function, because the last minute is well known.