No, this does not appear to be legal. Employers are allowed to round to the nearest few minutes (e.g. 5 minutes), but only if they do so in a neutral way which does not systematically reduce employee compensation. For example, in the situation you describe, rounding 8:31 and 4:34 to 8:30 and 4:35 respectively would be legal, since it is neutral, fair, and mathematically correct (that's how you would round to the nearest 5, mathematically speaking). However, rounding 8:31 and 4:34 to 8:30 and 4:30 would be improper, since it is biased against the employee (and mathematically invalid, and hence indefensible). Your employer may be violating the labor laws, from what you write, and you may therefore have a valid claim or complaint. You may wish to contact your state department of labor.