C++ :: Vector Of Keyframes For Animation - Calculate Index

I have vector of keyframes for animation:

struct Keyframe {
float time;// time in seconds
...
};
std::vector<Keyframe> vKeys;

And every loop i accumulate delta time to animate my object:

update(float deltaTime) {
timeRunning += deltaTime;
const float animationLen = 5.0f; // 5 seconds is total length of animation
...

To calculate current Key/index into vector i use:

float currTime = std::fmod(timeRunning, animationLen);
m_currKeyFrameInx = std::size_t(vKeys.size() * currTime / animationLen);

... use index

Can this logic fail?

Before i was using this:

float currTime = std::fmod(timeRunning, animationLen);
m_currKeyFrameInx = 0u;
for (std::size_t i = vKeys.size(); --i;) {
if (vKeys[i].time < currTime) {
m_currKeyFrameInx = i;
break;
}
}