[lawrenceA (OP)]

Using the vernal equinox as a marker I am sure we can tell the day of the year. From star patterns we can tell the time.

Is there any easy way or existing, free, software that will enable us to do so at any time?

If no software is available, how do we calc this?

[Beesriwara]

I am very glad to read this. It is very useful. I read and get more knowledge.

[evan_au]

The most convenient astral entity is the Sun.
The most useful app is the Sundial, which has the advantage that the batteries don't go flat.
But you do need to know the latitude, and the time of year can make a difference of up to 15 minutes (if we ignore daylight savings).
See: https://en.wikipedia.org/wiki/Sundial

If you are looking for an app on your smartphone, the most convenient astral constellation is the GPS satellites.
Every smartphone comes with an app that will tell the time, date and location, using GPS.
See: https://en.wikipedia.org/wiki/Global_Positioning_System