Flexi-time in some parts of the world usually refers to accumulated overtime hours that an employee can build up and exchange for the equivalent amount of time off. Basically, you need to keep track of time worked over a defined threshold. That’s no problem with Virtual TimeClock because you can create overtime rules to accommodate just about anything.
Some of the international customers I've chatted with recently track flexi-time monthly even though overtime is calculated on a daily or weekly basis. So when you run employee timecards for an entire month, the overtime will be calculated separately. This can then either be paid out or entered as a leave award. I recommend creating a new leave category called “Flexi”. You would just have to manually add the hours to Flexi every month for each employee and when they use the Flexi time, you would add it to their timecard as paid leave.