Is there any stipulation that sandhyavandanam must be done within a particular time? For example, can it be done before 10 PM? At times, during travels, sandhyavandanam cannot be performed at the appropriate time. Are there any atonements to follow when performed untimely?

Sandhyavandanam must be performed within the stipulated time. If it is not done within one and half hours of the stipulated time, atonements such as pranayamam, etc. must be done.
These days some perform at 8 PM or 9 PM; this is not that bad. But during long journeys, one is forced to perform a evening sandhyavandanam either early next dawn or morning. On such occasions, as atonement, an increased number of pranayamams are performed – 3 extra pranayamams are stipulated.