Three sandhya time.

The traditional timing is that the morning sandhyāvandanam should be when the sun is not above the level of eye. You can perform it anytime after midnight, but before the sun’s glory is fully spread in sky.

Noon mādhyāhnikam is when sun is around directly above the head so that you don’t have to slant yourself to watch it as you look up during the ritual.

Sāyaṃ sandhyāvandanam is when sun is above the eye or just at the eye level, and is about to set. You can chant it anytime before the sun sets fully.

If you are not able to keep up the timing, you might do the aghamarṣaṇam in the above timings, and save the upasthāna for your comfortable timing in morning, noon and evening. The sāvitrī you might mutter many times, does count for all those transgressions already, so no worries.

quora