In a sense, every distance can be converted into a duration or vice versa: simply multiply duration by the modal speed or multiply the distance by the modal pace. For example, every time can be multiplied by the speed of light in a vacuum and so be replaced by a distance. This is usually done with the invariant interval: ds² = c dt² – dr².
However, these distances are not really durations and these durations are not really distances. Such manipulations cannot change whether the variables were measured synchronously or asynchronously, which is an essential part of what the units represent. These conversions have their place but they do not show the symmetry of space and time.
What does show the symmetry of space and time is a switch of perspective so that what was the independent variable becomes the dependent variable and vice versa. This means every speed becomes to a pace and 3D space is mapped to 3D time, and vice versa.
It’s not the modal rate that makes space and time symmetric, though that does fit with the symmetry. It’s the arbitrary nature of the difference between space and time that make them symmetric. There is no inherent reason why 3D space makes more sense than 3D time or vice versa. They are symmetric.