in a convenient time vs at a convenient time

Some examples from the web:

• It's not a convenient time for me, Alex.
• I would've been happy to come out and visit you at a convenient time.
• Is this not a convenient time for you?
• - This was the most convenient time of day.
• Well, that's a pretty convenient time for a camera malfunction.
• Sweetheart, not really the most convenient time for me.
• You just show up at an incredibly convenient time.
• But I can come back at a more convenient time.
• I wonder if this might be a convenient time for me to learn?
• Okay, then I'll just tell Hector Matrick you'll call him back at a more convenient time.
• Do choose a more convenient time for quarrelling!
• The debate will take place at a more convenient time, under more favourable conditions.
• He said that he is willing to meet Prime Minister Sharon at a mutually convenient time.
• We're quite fatigued, so if it's not a bother, call back at a more convenient time.
• You two are just waiting for the convenient time to hand me back over to him.
• Or the killer could've put the body on ice until they found a more convenient time to dispose of it.
• Madam President, things rarely happen at convenient times and any crisis, by definition, is going to be inconvenient.
• ...at a more convenient time for the club?
• I will be very brief, Madam President. Moreover, I feel that this issue should be discussed in more detail, perhaps at a more convenient time.
• It's never been more than a... a convenient part-time job for you.

Some examples from the web:

• I would've been happy to come out and visit you at a convenient time.
• But I can come back at a more convenient time.
• Okay, then I'll just tell Hector Matrick you'll call him back at a more convenient time.
• The debate will take place at a more convenient time, under more favourable conditions.
• He said that he is willing to meet Prime Minister Sharon at a mutually convenient time.
• We're quite fatigued, so if it's not a bother, call back at a more convenient time.
• ...at a more convenient time for the club?
• I will be very brief, Madam President. Moreover, I feel that this issue should be discussed in more detail, perhaps at a more convenient time.
• Will it be possible to be admitted... at a more convenient time for the club?
• Even though the relinquishment of this clause is not linked solely to the preparation of this sale, there is no denying that it has come at a very convenient time for the owner.
• Does he knock on doors and offer to come back at a more convenient time?
• A poison... harmless when ingested, but at a time convenient to you, can be... remotely activated.
• It's not a convenient time for me, Alex.
• Is this not a convenient time for you?
• - This was the most convenient time of day.
• Well, that's a pretty convenient time for a camera malfunction.
• Sweetheart, not really the most convenient time for me.
• You just show up at an incredibly convenient time.
• I wonder if this might be a convenient time for me to learn?
• Do choose a more convenient time for quarrelling!