Earlier this week I was working on an issue in a plugin of ours, Order Delivery Date.
The issue was that date shown on the delivery calendar on WooCommerce checkout was shown as 1-1-1970.
This was rather strange. But I later realized that the minute field value that is passed in strtotime() is a single digit & there is no separator between hour & minute field. So if the time is 10:00, it was being passed in strtotime as 100.
Unless there is a colon separator between hour & minute, the timestamp will be returned blank when the minute is in single digits.
A safer option is to always put the separator when passing it to strtotime().