I've heard this one come up in a few contexts, and it seems Zig
Ziglar coined a similar phrase, but I first heard it about
investing: 'the best time to invest was 10 years ago, the second
best time is now' about the affect of compounding. However it also
comes up in the energy sector: 'the best time to invest in [green
energy/nuclear/stationary storage/etc] was 5 years ago, the second
best time...', as well as others and over time I've found it can
apply more generally to the day-to-day of life.
There are always those things we're behind on, and we feel on some
level guilty for: catching up with a friend, or engaging in a tough
conversation, or just a re-painting a stain on a wall; something
that we think 'if I'd done that X ago it would have been so much
better' and the weight of that pushes us to not do the thing: the
friend might be annoyed we haven't been in contact, or the situation
might get worse as the conversation wasn't had immediately, or the
stain might now be engrained and harder to shift. And so the longer
something has been left, the more it pushes us to leave it just that
bit longer. This proverb is about noticing that impulse, recognising
it for what it is, and deciding that although it may well be true
that you might have missed the best time to do something, the second
best time is available to you and you should take it.