I've seen a lot of games that use time as health but I don't think I've seen one use it directly as a currency... it's quite a unique take and it really makes that early game feel tense as you scramble to optimise your limited health without losing. There's definitely an interest concept here to expand, but I think the difficult design work is going to be keeping the player at low time for as long as possible to maintain that tension (late game you accumulate too much time). Also I don't know if it was intentional or just part of how you implemented the attack, but I actually love how there's a short window after I click that lets me drag the attack across an area of enemies and hit them all. It adds a weightiness to the attack that an instantaneous click to attack doesn't. It made me feel like I was getting away with something (always a great feeling in games!). Nurture this seed and see where it goes grows..!